As filed with the Securities and Exchange Commission on July 6, 2020

Registration No. 333-                

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM F-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

MEREO BIOPHARMA GROUP PLC

(Exact Name of Registrant as Specified in Its Charter)

Not Applicable

(Translation of Registrant’s Name into English)

Fourth Floor

One Cavendish Place

London W1G 0QF UK

Telephone: +44 33 3023 7300

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Mereo US Holdings Inc.

251 Little Falls Drive

Wilmington, DE 19808

Telephone: +1 302 636 5401

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copies to:

David S. Bakst

Mayer Brown LLP

1221 Avenue of the Americas

New York, New York 10020

Telephone: +1 212 506 2500

Approximate date of commencement of proposed sale to the public:

From time to time after the effective date of this Registration Statement.

If only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.    ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.    ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ☐

If this Form is a registration statement pursuant to General Instruction I.C. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.    ☐

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.C. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.    ☐

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933. Emerging growth company    ☒

If an emerging growth company that prepares its financial statements in accordance with U.S. GAAP, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards† provided pursuant to Section 7(a)(2)(B) of the Securities Act.    ☐

†The term “new or revised financial accounting standard” refers to any update issued by the Financial Accounting Standards Board to its Accounting Standards Codification after April 5, 2012.

The Registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act or until the registration statement shall become effective on such date as the SEC, acting pursuant to said Section 8(a), may determine.

The information in this preliminary prospectus is not complete and may be changed. This preliminary prospectus is not an offer to sell these securities and is not soliciting an offer to buy these securities in any state or jurisdiction where the offer or sale is not permitted.

103,098,851 American Depositary Shares

Representing 515,494,255 Ordinary Shares

This prospectus relates to the resale, by the selling shareholders identified in this prospectus, of up to an aggregate of 515,494,255 ordinary shares, nominal value £0.003 per share of Mereo BioPharma Group plc, equivalent to 103,098,851 American Depository Shares, or ADSs, consisting of (i) 89,144,623 ordinary shares, equivalent to 17,828,924 ADSs, issued to the selling shareholders pursuant to the June 2020 Private Placement and other ordinary shares previously held by certain selling shareholders prior to such private placement, (ii) 161,048,366 ordinary shares equivalent to 32,209,673 ADSs, which is the number of ordinary shares issuable upon the full exercise of warrants outstanding issued to investors in the June 2020 Private Placement, (iii) 253,048,551 ordinary shares equivalent to 50,609,710 ADSs, which is the number of ordinary shares issued upon the conversion of the convertible notes as of the date hereof and issuable upon the full conversion of the convertible notes outstanding as of the date hereof assuming the maximum amount of accrued interest for which additional ordinary shares are issuable upon conversion (including both accrued interest through the date hereof and accruing to the maturity date of the convertible notes assuming such notes remain outstanding until their maturity date), issued to investors in connection with the June 2020 Private Placement without regard to any ownership limitations, and (iv) 12,252,715 ordinary shares, equivalent to 2,450,543 ADSs, issued to Boxer Capital, LLC in a private placement on February 19, 2020.

The selling shareholders are identified in the table commencing on page 20. Each ADS represents five ordinary shares. No ADSs are being registered hereunder for sale by us. We will not receive any proceeds from the sale of the ADSs by the selling shareholders. All net proceeds from the sale of the ordinary shares represented by ADSs covered by this prospectus will go to the selling shareholders. However, we may receive the proceeds from any exercise of warrants in certain circumstances. See “Use of Proceeds.”

The selling shareholders may sell all or a portion of the ordinary shares represented by ADSs from time to time in market transactions through any market on which our ADSs are then traded, in negotiated transactions or otherwise, and at prices and on terms that will be determined by the then prevailing market price or at negotiated prices directly or through a broker or brokers, who may act as agent or as principal or by a combination of such methods of sale. See “Plan of Distribution”.

The ADSs may be evidenced by American Depositary Receipts (“ADRs”). Our ADSs trade on the Nasdaq Global Market (“Nasdaq”) under the symbol “MREO.” In addition, our ordinary shares trade on AIM, a market of the London Stock Exchange, under the symbol “MPH.”

We are both an “emerging growth company” and a “foreign private issuer” as defined under the Securities Act of 1933, as amended, and, as such, are subject to reduced public company reporting requirements. See “Prospectus Summary—Implications of Being an Emerging Growth Company and a Foreign Private Issuer” for additional information.

Investment in our ADSs involve significant risks. See “Risk Factors” beginning on page 7 of this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed on the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

Prospectus dated                , 2020

TABLE OF CONTENTS

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RISK FACTORS

Investing in our securities involves significant risks. Before making an investment decision, you should carefully consider the risks described under “Risk Factors” under Item 3.D. – “Risk Factors” in our most recent Annual Report on Form 20-F, or any updates in our Reports on Form 6-K, together with all of the other information appearing in this prospectus or incorporated by reference into this prospectus, in light of your particular investment objectives and financial circumstances. The risks so described are not the only risks facing us. Additional risks not presently known to us or that we currently deem immaterial may also impair our business operations. Our business, financial condition and results of operations could be materially adversely affected by any of these risks. The trading price of our securities could decline due to any of these risks, and you may lose all or part of your investment. The discussion of risks includes or refers to forward-looking statements; you should read the explanation of the qualifications and limitations on such forward-looking statements discussed elsewhere in this prospectus.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated into it contain statements that constitute forward-looking statements. Many of these forward-looking statements can be identified by the use of forward-looking words such as “anticipate,” “believe,” “could,” “estimate,” “expect,” “intend,” “plan,” “potential” and “should,” among others.

Forward-looking statements appear in a number of places in this prospectus and the documents incorporated into it and include, but are not limited to, statements regarding our intent, belief, or current expectations. Forward-looking statements are based on our management’s beliefs and assumptions and on information currently available to our management. Such statements are subject to substantial risks and uncertainties, and actual results may differ materially from those expressed or implied in the forward-looking statements due to various important factors, including, but not limited to, those identified under “Risk Factors.” In light of the significant uncertainties in these forward-looking statements, you should not regard these statements as a guarantee by us or any other person that we will achieve our objectives and plans in any specified time frame, or at all.

Forward-looking statements include, but are not limited to, statements about:

Forward-looking statements speak only as of the date they are made, and we do not undertake any obligation to update them in light of new information or future developments or to release publicly any revisions to these statements in order to reflect later events or circumstances or to reflect the occurrence of unanticipated events.

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You should read this prospectus and the documents that we incorporate by reference in this prospectus and have filed as exhibits to the registration statement, of which this prospectus is a part, completely and with the understanding that our actual future results may be materially different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

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CAPITALIZATION

The table below sets forth our capitalization, cash and short-term deposits as of December 31, 2019, the latest date for which we have audited financial statements and the information below available to us. Because we will not be receiving any proceeds from the sale of any ordinary shares by the selling shareholders, the capitalization information below is not adjusted to reflect such sales. The following information should be read in conjunction with the consolidated financial statements and related notes incorporated by reference in this prospectus. For more details on how you can obtain the documents incorporated by reference in this prospectus, see “Where You Can Find More Information” and “Incorporation by Reference”.

For the convenience of the reader, we have translated pound sterling amounts in the table below into U.S. dollars at an exchange rate of £0.7692 to US$1.00, the exchange rate for pound sterling on December 31, 2019. These translations should not be considered representations that any such amounts have been, could have been or could be converted into U.S. dollars at that or any other exchange rate as of that or any other date.

The cash, short-term deposits and number of ordinary shares indicated as issued and outstanding above is based on 97,959,622 ordinary shares outstanding as of December 31, 2019, and excludes:

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As of December 31, 2019, our net tangible book value was negative £4.2 million and as a result we are not presenting dilution in this prospectus because we believe the presentation would not be meaningful and would not show material dilution to investors in the offering.

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USE OF PROCEEDS

We will not receive any proceeds from the sale of the ordinary shares represented by ADSs by the selling shareholders. We expect that the selling shareholders will sell their ordinary shares represented by ADSs as described under “Plan of Distribution.”

We may receive proceeds from the exercise of the warrants issued to the selling shareholders in the June 2020 Private Placement upon such exercise. If all of the warrants were exercised for cash in full, the proceeds would be approximately £56 million ($70 million). Holders of warrants have the right to choose a cashless exercise option which if chosen would result in a lower amount of proceeds to us from the warrant exercise than the exercise for cash in full. We intend to use the net proceeds of such warrant exercise, if any, for research and development, general and administrative expenses, and for working capital purposes. Pending such uses, we intend to invest the net proceeds in short-term, interest-bearing, investment grade securities or as otherwise pursuant our customary investment policies. We can make no assurances that any of the warrants will be exercised, the quantity which will be exercised or in the period in which they will be exercised.

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MATERIAL TAX CONSIDERATIONS

Material U.S. Federal Income Tax Considerations

The following are material U.S. federal income tax consequences to the U.S. Holders (as defined below) of purchasing, owning and disposing of the ADSs and ordinary shares acquired pursuant to this offering, but it does not purport to be a comprehensive description of all tax considerations that may be relevant to a particular person’s decision to acquire the securities. This discussion applies only to a U.S. Holder that acquires ADSs in this offering and holds the ADSs or ordinary shares as capital assets for U.S. federal income tax purposes. In addition, it does not describe all of the tax consequences that may be relevant in light of the U.S. Holder’s particular circumstances, including any estate, gift, alternative minimum or Medicare contribution tax consequences and any tax consequences applicable to U.S. Holders subject to special rules, such as:

If a partnership (or other entity that is classified as a partnership for U.S. federal income tax purposes) owns the ADSs or ordinary shares, the U.S. federal income tax treatment of a partner will generally depend on the status of the partner and the activities of the partner and the partnership. Partnerships owning the ADSs or ordinary shares and partners in such partnerships should consult their tax advisers as to the particular U.S. federal income tax consequences of owning and disposing of the ADSs or ordinary shares.

Persons that own or are deemed to own 10% or more of our stock by vote or value should consult their tax advisers regarding the application of the “controlled foreign corporation” rules to their ownership of our ADSs or ordinary shares.

This discussion is based on the Code, administrative pronouncements, judicial decisions, and final, temporary and proposed Treasury regulations, all as of the date hereof, any of which is subject to change, possibly with retroactive effect.

As used herein, a “U.S. Holder” is a person that, for U.S. federal income tax purposes, is a beneficial owner of our ADSs or ordinary shares and is:

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U.S. Holders should consult their tax advisers concerning the U.S. federal, state, local and non-U.S. tax consequences of owning and disposing of our ADSs or ordinary shares in their particular circumstances.

For U.S. federal income tax purposes, a beneficial owner of our ADSs generally will be treated as the owner of the underlying ordinary shares represented by such ADSs. Accordingly, gain or loss will generally not be recognized if a U.S. Holder exchanges our ADSs for the underlying ordinary shares.

Passive Foreign Investment Company Rules

Special U.S. tax rules apply to U.S. Holders of stock in companies that are considered to be PFICs. In general, a non-U.S. corporation will be a passive foreign investment company (“PFIC”) for any taxable year in which (i) 75% or more of its gross income consists of passive income (the “income test”) or (ii) 50% or more of the value of its assets (generally determined on a quarterly average basis) consists of assets that produce, or are held for the production of, passive income (the “asset test”). For purposes of the above calculations, a non-U.S. corporation that directly or indirectly owns at least 25% by value of the shares of another corporation is treated as if it held its proportionate share of the assets of the other corporation and received directly its proportionate share of the income of the other corporation. Passive income generally includes interest, dividends, gains from certain property transactions, rents and royalties (other than certain rents or royalties derived in the active conduct of a trade or business). Cash is a passive asset for PFIC purposes. Goodwill (the value of which may be determined by reference to the company’s market capitalization) is treated as an active asset to the extent attributable to activities intended to produce active income.

Based on our gross income, the average value of our assets, including goodwill, and the nature of the current stage of our business, we believe we were a PFIC for the year ended December 31, 2019. There can be no assurance regarding our PFIC status for the current taxable year or any particular year in the future because PFIC status is factual in nature, depends upon factors not wholly within our control, generally cannot be determined until the close of the taxable year in question and is determined annually. Accordingly, U.S. Holders should invest in our ADSs only if they are willing to bear the U.S. federal income tax consequences associated with investments in PFICs.

We provide the information necessary for a U.S. Holder to make a qualifying electing fund election (“QEF Election”) with respect to us and we will also use our best efforts to cause each Lower-tier PFIC (as defined below) that we control to provide such information. We intend to provide this information for any taxable year during which our only income is interest income or income from financial investments and for any other taxable year for which we determine that we were a PFIC. However, no assurance can be given that such QEF information will be available for any Lower-tier PFIC that we do not wholly-own. We will post the information necessary to make QEF Elections on our website. If we are a PFIC for any taxable year, the consequences to any U.S. Holder will depend in part on whether the U.S. Holder makes a valid QEF Election or mark-to-market election as described below.

If we are a PFIC for any taxable year and any of our non-U.S. subsidiaries or other companies in which we own equity interests were also a PFIC (any such entity, a “Lower-tier PFIC”), U.S. Holders

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would be deemed to own a proportionate amount (by value) of the shares of each Lower-tier PFIC and would be subject to U.S. federal income tax according to the rules described in the subsequent paragraph on (i) certain distributions by a Lower-tier PFIC and (ii) dispositions of shares of Lower-tier PFICs, in each case as if the U.S. Holders held such shares directly, even though the U.S. Holders had not received the proceeds of those distributions or dispositions.

Generally, if we were a PFIC for any taxable year during which a U.S. Holder holds our ADSs or ordinary shares and the U.S. Holder does not make a valid QEF Election or a mark-to-market election (described below), gain recognized upon a disposition (including, under certain circumstances, a pledge) of our ADSs or ordinary shares by the U.S. Holder will be allocated ratably over the U.S. Holder’s holding period for such ADSs or ordinary shares. The amounts allocated to the taxable year of disposition and to years before we became a PFIC will be taxed as ordinary income. The amount allocated to each other taxable year will be subject to tax at the highest rate in effect for that taxable year for individuals or corporations, as appropriate, and an interest charge will be imposed on the resulting tax liability for each relevant taxable year. Further, to the extent that any distribution received by a U.S. Holder on our ADSs or ordinary shares exceeds 125% of the average of the annual distributions received on such securities during the preceding three years or the U.S. Holder’s holding period, whichever is shorter (an “excess distribution”), such excess distribution will be subject to taxation in the same manner. If we are a PFIC for any taxable year during which a U.S. Holder owns our ADSs or ordinary shares, we will generally continue to be treated as a PFIC with respect to such U.S. Holder for all succeeding years during which such U.S. Holder owns our ADSs or ordinary shares, even if we cease to meet the threshold requirements for PFIC status. If we are a PFIC for any taxable year but cease to be PFIC for subsequent years, U.S. Holders should consult their tax advisers regarding the advisability of making a “deemed sale” election that would allow them to eliminate the continuing PFIC status under certain circumstances.

To avoid the foregoing rules, a U.S. Holder can make a QEF Election to treat us and each Lower-tier PFIC as a qualified electing fund in the first taxable year that the entity is treated as a PFIC with respect to the U.S. Holder. A U.S. Holder must make the QEF Election for each PFIC by attaching a separate properly completed IRS Form 8621 for that PFIC to the U.S. Holder’s timely filed U.S. federal income tax return. A U.S. Holder making a QEF election other than for the first taxable year in which it owns (or is treated as owning) an equity interest in a PFIC would continue to be subject to the rules described in the preceding paragraph with respect to such PFIC, unless the U.S. Holder makes a “deemed sale” election with respect to the PFIC and recognizes gain taxed under the general PFIC rules described above with respect to the PFIC stock’s appreciation before the year for which the QEF Election is made.

If a U.S. Holder makes a QEF Election with respect to a PFIC, the U.S. Holder will be taxed on its pro rata share of the PFIC’s ordinary earnings and net capital gain (at ordinary income and capital gain rates, respectively) for each taxable year that the entity is a PFIC. If a U.S. Holder makes a QEF Election with respect to us, any distributions we pay out of our earnings and profits that were previously included in the U.S. Holder’s income under the QEF Election would not be taxable to the U.S. Holder. A U.S. Holder will increase its tax basis in its ADSs or ordinary shares by an amount equal to any income included under the QEF Election and will decrease its tax basis by any amount distributed on the ADSs or ordinary shares that is not included in the U.S. Holder’s income. In addition, a U.S. Holder will recognize capital gain or loss on the disposition of ADSs or ordinary shares in an amount equal to the difference between the amount realized and the U.S. Holder’s adjusted tax basis in the ADSs or ordinary shares, as determined in U.S. dollars. A. U.S. Holder will not be taxed on the ordinary income and net capital gain under the QEF rules for any year that we are not a PFIC.

Based on the nature of our expected income, the expected composition of our assets, and our business prospects, we do not currently expect to have significant ordinary earnings or net capital gain

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in any taxable year in which we may be a PFIC. However, it is difficult to predict the nature and composition of our income and assets and the value of our assets in light of the volatile nature of earnings patterns of emerging pharmaceutical or biotechnology companies such as us. Accordingly, U.S. Holders should note that if they make QEF Elections with respect to us and our subsidiaries, they may be required to pay U.S. federal income tax with respect to their ADSs or ordinary shares for any taxable year in which we have a positive amount of earnings or net capital gains even if we do not make any distributions in such year. U.S. Holders should consult their tax advisers regarding the advisability of making QEF Elections in their particular circumstances.

Alternatively, if we are a PFIC for any taxable year and if our ADSs or ordinary shares are “regularly traded” on a “qualified exchange,” a U.S. Holder could make a mark-to-market election that will result in tax treatment different from the general tax treatment described in the two preceding paragraphs. Our ADSs and/or ordinary shares will be treated as “regularly traded” in any calendar year in which more than a de minimis quantity of the ADSs and/or ordinary shares are traded on a qualified exchange on at least 15 days during each calendar quarter. NASDAQ, on which the ADSs are listed, is a qualified exchange for this purpose. The Internal Revenue Service has not identified specific non-U.S. exchanges that are “qualified” for this purpose. If a U.S. Holder makes a valid mark-to-market election, the U.S. Holder generally will recognize as ordinary income any excess of the fair market value of its ADSs or ordinary shares at the end of each taxable year over the adjusted tax basis of such ADSs or ordinary shares, and will recognize an ordinary loss in respect of any excess of the adjusted tax basis of its ADSs or ordinary shares over their fair market value at the end of the taxable year (but only to the extent of the net amount of income previously included as a result of the mark-to-market election). If a U.S. Holder makes the election, the U.S. Holder’s tax basis in our ADSs or ordinary shares will be adjusted to reflect these income or loss amounts. Any gain recognized on the sale or other disposition of our ADSs or ordinary shares in a year in which we are a PFIC will be treated as ordinary income and any loss will be treated as an ordinary loss (but only to the extent of the net amount of income previously included as a result of the mark-to-market election). If a valid mark-to-market election is made for any year in which we are a PFIC, distributions will be treated as described below under “—Taxation of Distributions” except that the preferential tax rates on dividends paid to non-corporate U.S. Holders will not apply. U.S. Holders will not be able to make a mark-to-market election with respect to Lower-tier PFICs, if any. U.S. Holders should consult their tax advisers as to the availability and desirability of a mark-to-market election in their particular circumstances if we are a PFIC for any taxable year.

If a U.S. Holder owns our ADSs or ordinary shares during any year in which we are a PFIC, the U.S. Holder generally will be required to file annual reports on IRS Form 8621 (or any successor form) with respect to us and any Lower-tier PFIC, generally with the U.S. Holder’s U.S. federal income tax return for that year. U.S. Holders should consult their tax advisers regarding our PFIC status for any taxable year and the potential application of the PFIC rules to an investment in our ADSs or ordinary shares.

Taxation of Distributions

This discussion under “—Taxation of Distributions” is subject to the PFIC rules described in “—Passive Foreign Investment Company Rules” above. Distributions paid on ADSs or ordinary shares, other than certain pro rata distributions of our ordinary shares, will be treated as dividends to the extent paid out of our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Distributions in excess of our current and accumulated earnings and profits will be treated first as a tax-free return of capital to the extent of the U.S. Holder’s basis in the ADSs or ordinary shares and then as capital gain. For any taxable year in which we do not maintain calculations of our earnings and profits under U.S. federal income tax principles, it is expected that any distributions generally will be reported to U.S. Holders as dividends. Dividends will not be eligible for the dividends-

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received deduction generally available to U.S. corporations under the Code. Subject to applicable limitations, dividends paid to certain non-corporate U.S. Holders may be eligible for taxation at a preferential tax rate provided that we were not a PFIC for the taxable year in which the dividend is paid or the prior taxable year. Non-corporate U.S. Holders should consult their tax advisers regarding the availability of this preferential rate in the light of the discussion in “—Passive Foreign Investment Company Rules” above and in their particular circumstances.

If dividend payments in respect of our ADSs or ordinary shares are made in a currency other than the U.S. dollar, the amount of the dividend distribution that a U.S. Holder must include in income will be the U.S. dollar value of the payments made in such other currency, determined at the spot U.S. dollar exchange rate on the date the dividend distribution is includible in income, regardless of whether the payment is in fact converted into U.S. dollars. Generally, if the foreign currency received as a dividend is not converted into U.S. dollars on the date of receipt, any gain or loss resulting from currency exchange fluctuations during the period from the date the dividend payment is includible in income to the date the payment is actually converted into U.S. dollars will be treated as ordinary income or loss and will not be eligible for the special tax rate applicable to qualified dividend income. The gain or loss generally will be income or loss from sources within the United States for foreign tax credit limitation purposes. U.S. Holders are urged to consult their tax advisors regarding the tax consequences of receiving, converting or disposing of any non-U.S. currency, received or deemed received as dividends on our ADSs or ordinary shares or on the sale or retirement of an ADS or an ordinary share.

Dividends will be included in a U.S. Holder’s income on the date of the U.S. Holder’s, or in the case of our ADSs, the depositary’s, receipt. Dividends generally will be income from non-U.S. sources, which may be relevant in calculating a U.S. Holder’s foreign tax credit limitation. Subject to certain conditions and limitations, non-U.S. tax withheld, if any, on dividends may be deducted from such U.S. Holder’s taxable income or credited against such U.S. Holder’s U.S. federal income tax liability. The limitation on foreign taxes eligible for credit is calculated separately with respect to specific classes of income. For this purpose, dividends that we distribute generally should constitute “passive category income,” or, in the case of certain U.S. Holders, “general category income.” A foreign tax credit for foreign taxes imposed on distributions may be denied if a U.S. Holder does not satisfy certain minimum holding period requirements. The rules relating to the determination of the foreign tax credit are complex, and U.S. Holders are urged to consult their tax advisors to determine whether and to what extent such U.S. Holder will be entitled to a foreign tax credit.

Sale or Other Taxable Disposition

Except as described under “—Passive Foreign Investment Company Rules” above, a U.S. Holder will generally recognize capital gain or loss on a sale or other taxable disposition of our ADSs or ordinary shares in an amount equal to the difference between the amount realized on the sale or disposition and the U.S. Holder’s tax basis in the ADSs or ordinary shares disposed of, in each case as determined in U.S. dollars. A U.S. Holder’s initial tax basis in the ordinary shares or ADSs will generally equal the cost of such ordinary shares or ADSs. If a U.S. Holder used foreign currency to purchase the ordinary shares or ADSs, the cost of the ordinary shares or ADSs will be the U.S. dollar value of the foreign currency purchase price on the date of purchase, translated at the spot rate of exchange on that date. Any such gain or loss will be long-term capital gain or loss if at the time of the sale or disposition the U.S. Holder has owned our ADSs or ordinary shares for more than one year. Long-term capital gains recognized by non-corporate U.S. Holders may be subject to a tax rate that is lower than the rate applicable to ordinary income. The deductibility of capital losses is subject to limitations. Any capital gain or loss recognized upon the sale or disposition of ADSs or ordinary shares will generally be treated as U.S.-source income for foreign tax credit limitation purposes. U.S. Holders that sell the ADSs or ordinary shares for an amount denominated in a currency other than the U.S. dollar should

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consult their tax advisers regarding any potential foreign currency gain or loss that may have to be recognized.

Information Reporting and Backup Withholding

In general, payments of dividends and proceeds from the sale or other disposition of our ADSs or ordinary shares that are made within the United States or through certain U.S.-related financial intermediaries may be subject to information reporting and backup withholding, unless (i) in the case of information reporting, the U.S. Holder is a corporation or other “exempt recipient” and (ii) in the case of backup withholding, the U.S. Holder provides a correct taxpayer identification number and certifies that it is not subject to backup withholding. Backup withholding is not an additional tax. The amount of any backup withholding from a payment to a U.S. Holder will be allowed as a credit against the U.S. Holder’s U.S. federal income tax liability and may entitle it to a refund, provided that the required information is timely furnished to the IRS.

Information with Respect to Foreign Financial Assets

Certain U.S. Holders who are individuals (or certain specified entities) may be required to report information relating to their ownership of our ADSs or ordinary shares, or non-U.S. accounts through which our ADSs or ordinary shares are held, subject to certain exceptions. Penalties and potential other adverse tax consequences may be imposed if a U.S. Holder is required to submit such information to the IRS and fails to do so. U.S. Holders should consult their tax advisers regarding their reporting obligations with respect to our ADSs or ordinary shares.

Material United Kingdom Tax Considerations

The following is a description of the material U.K. tax considerations relating primarily to the ownership and disposal of our ADSs by the U.S. Holders described above. The U.K. tax comments set out below are based on current U.K. tax law as applied in England and Wales, and HMRC practice (which may not be binding on HMRC) as at the date of this summary, both of which are subject to change, possibly with retrospective effect. They are intended as a general guide and, save where otherwise stated, only apply to you if you are not resident in the U.K. for U.K. tax purposes and do not hold our ADSs for the purposes of a trade, profession or vocation that you carry on in the U.K. through a branch, agency or permanent establishment in the U.K. and if you hold our ADSs as an investment for U.K. tax purposes and are not subject to special rules.

This summary does not address all possible tax consequences relating to an investment in our ADSs. In particular it does not cover the U.K. inheritance tax consequences of holding our ADSs. It assumes that DTC has not made an election under section 97A(1) of the Finance Act 1986. It assumes that we do not (and will not at any time) derive 75% or more of our qualifying asset value, directly or indirectly, from U.K. land, and that we are and remain solely resident in the U.K. for tax purposes. This summary is for general information only and is not intended to be, nor should it be considered to be, legal or tax advice to any particular holder. Holders of our ADSs are strongly urged to consult their tax advisers in connection with the U.K. tax consequences of their investment in our ADSs.

U.K. Taxation of Dividends

Mereo will not be required to withhold amounts for or on account of U.K. tax at source when paying a dividend in respect of its ordinary shares.

Holders who hold our ADSs as an investment, who are not resident in the U.K. for U.K. tax purposes and who do not hold their ADSs in connection with any trade, profession or vocation carried on by them in the U.K. through a branch, agency or permanent establishment in the U.K. should not be subject to U.K. tax in respect of any dividends on our ordinary shares.

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U.K. Taxation of Capital Gains

An individual holder who is not resident in the U.K. for U.K. tax purposes should not be liable to U.K. capital gains tax on capital gains realized on the disposal of their ADSs unless such holder carries on a trade, profession or vocation in the U.K. through a branch or agency in the U.K. to which ADSs are attributable.

Any such individual holder of our ADSs who is temporarily non-resident for U.K. tax purposes will, in certain circumstances, become liable to U.K. tax on capital gains in respect of gains realized while they were not resident in the U.K.

A corporate holder of our ADSs which is not resident in the U.K. for U.K. tax purposes should not be liable for U.K. corporation tax on chargeable gains realized on the disposal of our ADSs unless it carries on a trade in the U.K. through a permanent establishment in the U.K. to which our ADSs are attributable.

Stamp Duty and Stamp Duty Reserve Tax

The following statements apply to all holders, regardless of their jurisdiction of tax residence.

It is assumed for the purposes of the following statements that all transfers or, or agreements to transfer, our ordinary shares are only made at times when (i) our ordinary shares are admitted to trading on AIM but are not listed on any market (with the term “listed” being construed in accordance with section 99A of the Finance Act 1986); and (ii) AIM continues to be accepted as a “recognized growth market” (as construed in accordance with section 99A of the Finance Act 1986). Holders of our ADSs who propose to transfer, or agree to transfer, our ordinary shares during such time as these conditions are not met (including during any period between the creation and issue of our ADSs and the admission to trading of our ordinary shares on AIM) are strongly urged to obtain their own advice.

No stamp duty is payable on the issue of our ordinary shares into a depositary receipt system (such as, Mereo understands, that operated by Citibank) or a clearance service (such as, Mereo understands, DTC). No stamp duty reserve tax (“SDRT”) should be payable on the issue of our ordinary shares into a depositary receipt system or a clearance service. Accordingly, no stamp duty or SDRT should be payable on the creation and issue of our ADSs pursuant to the issue of our ordinary shares to Citibank’s custodian.

No stamp duty or SDRT should be payable on transfers of, or agreements to transfer, our ordinary shares into a depositary receipt system or a clearance service.

No SDRT or stamp duty should be payable on paperless transfers of, or agreements to transfer, our ADSs through the facilities of DTC.

No stamp duty should be payable on a written instrument transferring, or a written agreement to transfer, our ADSs provided the instrument or agreement is executed and remains at all times outside the U.K. No SDRT should be payable in respect of agreements to transfer our ADSs.

No stamp duty or SDRT should be payable on transfers of, or agreements to transfer, our ordinary shares outside of a depositary receipt system or a clearance service.

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SELLING SHAREHOLDERS

The ordinary shares which may be represented by ADSs being offered by the selling shareholders include (i) ordinary shares which may be represented by ADSs, (ii) ordinary shares which may be represented by ADSs issuable upon exercise of warrants, or (iii) ordinary shares issuable upon conversion of the convertible notes (including interest payable thereunder in accordance with the terms thereof), in each case, issued to the selling shareholders in connection with the June 2020 Private Placement and additional ordinary shares held by certain selling shareholders prior thereto. For additional information regarding the issuance of those securities, see “Prospectus Summary – June 2020 Private Placement” above. We are registering the ordinary shares which may be represented by ADSs in order to permit the selling shareholders to offer these securities for resale from time to time. Other than with respect to (i) Aspire Capital Fund, LLC, that was an existing shareholder prior to the June 2020 Private Placement and is party to a financing arrangement with us to purchase up to $25 million of our ADSs and purchased additional securities in the June 2020 Private Placement, (ii) Boxer Capital, LLC that was an existing shareholder prior to the June 2020 Private Placement and purchased additional securities in the June 2020 Private Placement, and except for the securities issued to the selling shareholders in the June 2020 Private Placement, the selling shareholders have not had any material relationship with us within the past three years.

The table below lists the selling shareholders and other information regarding the beneficial ownership of the ordinary shares which may be represented by ADSs by each of the selling shareholders. The second column lists the number of ordinary shares expressed as ADS equivalents beneficially owned by each selling shareholder, based on its ownership of ordinary shares or ADSs, as of July 6, 2020, excluding ordinary shares issuable upon exercise of the warrants and conversion of convertible notes issued in the June 2020 Private Placement and held by the selling shareholders, the exercise or conversion of which is subject to a beneficial ownership limitation of 9.99% of our outstanding ordinary shares as described below. The fourth column lists the maximum number of ordinary shares expressed as ADS equivalents being offered in this prospectus by the selling shareholders on that basis. The fifth and sixth columns list the amount of ordinary shares expressed as ADS equivalents owned after the offering, by number and percentage, assuming in both cases the sale of all of the securities offered by the selling shareholders pursuant to this prospectus, and giving effect to the contractual ownership limitations described below.

Under the terms of the warrants and convertible notes issued in the June 2020 Private Placement, a selling shareholder may not exercise the warrants or convert the convertible notes to the extent such exercise or conversion would cause such selling shareholder, together with its affiliates, to beneficially own a number of ordinary shares which would exceed 9.99% of our then outstanding ordinary shares following such exercise or conversion, excluding for purposes of such determination ordinary shares not yet issuable upon exercise of the warrants which have not been exercised or conversion of convertible notes not yet converted. This percentage limit may be increased or decreased in accordance with the provisions of the warrant instrument and convertible note instrument. The information in the table below regarding ownership of ordinary shares and ADSs reflects these ownership limitations. The selling shareholders may sell all, some or none of their ordinary shares or ADSs in this offering. See “Plan of Distribution.”

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PLAN OF DISTRIBUTION

We are registering the ordinary shares represented by ADSs issued and the ordinary shares represented by ADSs issuable upon exercise of the warrants and conversion of the convertible notes issued in the June 2020 Private Placement and held by certain selling shareholder prior thereto to permit the resale of these ordinary shares represented by ADSs by the holders of these ADSs from time to time after the date of this prospectus. We will not receive any of the proceeds from the sale by the selling shareholders of the ordinary shares represented by ADSs other than proceeds from the exercise of the warrants. We will bear all fees and expenses incident to our obligation to register the ordinary shares represented by ADSs.

The selling shareholders may sell all or a portion of the ordinary shares represented by ADSs beneficially owned by them and offered hereby from time to time directly or through one or more underwriters, broker-dealers or agents. If the ordinary shares represented by ADSs are sold through underwriters or broker-dealers, the selling shareholders will be responsible for underwriting discounts or commissions or agent’s commissions. The ordinary shares represented by ADSs may be sold in one or more transactions at fixed prices, at prevailing market prices at the time of the sale, at varying prices determined at the time of sale, or at negotiated prices. These sales may be effected in transactions, which may involve crosses or block transactions,

If the selling shareholders effect such transactions by selling ordinary shares represented by ADSs to or through underwriters, broker-dealers or agents, such underwriters, broker-dealers or agents may receive commissions in the form of discounts, concessions or commissions from the selling shareholders or commissions from purchasers of the ordinary shares represented by ADSs for whom they may act as agent or to whom they may sell as principal (which discounts, concessions or commissions as to particular underwriters, broker-dealers or agents may be in excess of those customary in the types of transactions involved). In connection with sales of ordinary shares represented by ADSs or otherwise, the selling shareholders may enter into hedging transactions with broker-dealers, which may in turn engage in short sales of the ordinary shares represented by ADSs in

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the course of hedging in positions they assume. The selling shareholders may also sell ordinary shares represented by ADSs short and deliver ordinary shares represented by ADSs covered by this prospectus to close out short positions and to return borrowed shares in connection with such short sales. The selling shareholders may also loan or pledge ordinary shares represented by ADSs to broker-dealers that in turn may sell such shares.

The selling shareholders may pledge or grant a security interest in some or all of the warrants or convertible notes owned by them and, if they default in the performance of their secured obligations, the pledgees or secured parties may offer and sell the ordinary shares represented by ADSs from time to time pursuant to this prospectus or any amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act of 1933, as amended, amending, if necessary, the list of selling shareholders to include the pledgee, transferee or other successors in interest as selling shareholders under this prospectus. The selling shareholders also may transfer and donate the ordinary shares represented by ADSs in other circumstances in which case the transferees, donees, pledgees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.

The selling shareholders and any broker-dealer participating in the distribution of the ordinary shares represented by ADSs may be deemed to be “underwriters” within the meaning of the Securities Act, and any commission paid, or any discounts or concessions allowed to, any such broker-dealer may be deemed to be underwriting commissions or discounts under the Securities Act. At the time a particular offering of the ordinary shares represented by ADSs is made, a prospectus supplement, if required, will be distributed which will set forth the aggregate amount of ordinary shares represented by ADSs being offered and the terms of the offering, including the name or names of any broker-dealers or agents, any discounts, commissions and other terms constituting compensation from the selling shareholders and any discounts, commissions or concessions allowed or reallowed or paid to broker-dealers.

There can be no assurance that any selling shareholder will sell any or all of the ordinary shares represented by ADSs registered pursuant to the registration statement, of which this prospectus forms a part.

The selling shareholders and any other person participating in such distribution will be subject to applicable provisions of the Exchange Act, and the rules and regulations thereunder, including, without limitation, Regulation M of the Exchange Act, which may limit the timing of purchases and sales of any of the ordinary shares represented by ADSs by the selling shareholders and any other participating person. Regulation M may also restrict the ability of any person engaged in the distribution of the ordinary shares represented by ADSs to engage in market-making activities with respect to the ordinary shares represented by ADSs. All of the foregoing may affect the marketability of the ordinary shares represented by ADSs and the ability of any person or entity to engage in market-making activities with respect to the ordinary shares represented by ADSs.

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EXPENSES OF THE OFFERING

We estimate that our expenses in connection with the offering will be as follows:

All amounts in the table are estimates except the SEC registration fee. We will pay all of the expenses referred to above.

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LEGAL MATTERS

Mayer Brown LLP has passed upon certain legal matters regarding the securities offered hereby. The validity of the ordinary shares underlying the ADSs to be offered pursuant to this prospectus will be passed upon for us by Mayer Brown International LLP.

EXPERTS

The consolidated financial statements of Mereo BioPharma Group plc appearing in Mereo BioPharma Group plc’s Annual Report (Form 20-F) for the year ended December 31, 2019, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

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SERVICE OF PROCESS AND ENFORCEMENT OF LIABILITIES

We are incorporated and currently existing under the laws of England and Wales. In addition, most of our directors and officers reside outside of the United States and most of our assets are located outside of the United States. As a result, it may be difficult for investors to effect service of process in the United States on us or those persons or to enforce in the United States judgments obtained in United States courts against us or those persons based on the civil liability or other provisions of the United States securities laws or other laws.

In addition, uncertainty exists as to whether the courts of England and Wales would:

There is currently no treaty between (i) the United States and (ii) England and Wales providing for reciprocal recognition and enforcement of judgments of United States courts in civil and commercial matters, although the United States and the United Kingdom are both parties to the New York Convention on the Recognition and Enforcement of Foreign Arbitral Awards. A final judgment for the payment of money rendered by any general or state court in the United States based on civil liability, whether or not predicated solely upon the United States securities laws, will not be automatically enforceable in England and Wales. Any final and conclusive monetary judgment for a definite sum obtained against us in United States courts will be treated by the courts of England and Wales as a cause of action in itself and sued upon as a debt at common law so that no retrial of the issues will be necessary, provided that:

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Whether these requirements are met in respect of a judgment based upon the civil liability provisions of the United States securities laws, including whether the award of monetary damages under such laws would constitute a penalty, is an issue for the English court making such decision.

Subject to the foregoing, investors may be able to enforce in England and Wales judgments in civil and commercial matters that have been obtained from U.S. federal or state courts. Nevertheless, we cannot assure you that those judgments will be recognized or enforceable in England and Wales.

If an English court gives judgment for the sum payable under a U.S. judgment, the English judgment will be enforceable by methods generally available for this purpose. These methods generally permit the English court discretion to prescribe the manner of enforcement. In addition, it may not be possible to obtain an English judgment or to enforce that judgment if the judgment debtor is or becomes subject to any insolvency or similar proceedings, or if the judgment debtor has any set-off or counterclaim against the judgment creditor. Also note that, in any enforcement proceedings, the judgment debtor may raise any counterclaim that could have been brought if the action had been originally brought in England unless the subject of the counterclaim was in issue and denied in the U.S. proceedings.

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WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC a registration statement (including amendments and exhibits to the registration statement) on Form F-3 under the Securities Act. This prospectus, which is part of the registration statement, does not contain all of the information set forth in the registration statement and the exhibits and schedules to the registration statement. For further information, we refer you to the registration statement and the exhibits and schedules filed as part of the registration statement. If a document has been filed as an exhibit to the registration statement, we refer you to the copy of the document that has been filed. Each statement in this prospectus relating to a document filed as an exhibit is qualified in all respects by the filed exhibit.

We are subject to the informational requirements of the Exchange Act. Accordingly, we are required to file reports and other information with the SEC, including annual reports on Form 20-F and reports on Form 6-K. The SEC maintains an Internet website that contains reports, proxy and information statements, and other information about issuers, like us, that file electronically with the SEC. The address of that website is www.sec.gov.

As a foreign private issuer, we are exempt under the Exchange Act from, among other things, the rules prescribing the furnishing and content of proxy statements, and our board members, executive officers, and principal shareholders are exempt from the reporting and short-swing profit recovery provisions contained in Section 16 of the Exchange Act. In addition, we will not be required under the Exchange Act to file periodic reports and financial statements with the SEC as frequently or as promptly as U.S. companies whose securities are registered under the Exchange Act.

We maintain a corporate website at https://www.mereobiopharma.com. Information contained on, or that can be accessed through, our website does not constitute a part of this prospectus and our website address is included in this prospectus as an inactive textual reference only.

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INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” information into this prospectus, which means that we can disclose important information to you by referring you to other documents which we have filed or will file with the SEC. We are incorporating by reference in this prospectus the documents listed below and all amendments or supplements we may file to such documents, as well as any future filings we may make with the SEC on Form 20-F under the Exchange Act before the time that all of the securities offered by this prospectus have been sold or de-registered:

In addition, any reports on Form 6-K submitted to the SEC by us pursuant to the Exchange Act after the date of the initial registration statement and prior to effectiveness of the registration statement that we specifically identify in such forms as being incorporated by reference into the registration statement of which this prospectus forms a part and all subsequent annual reports on Form 20-F filed after the effective date of this registration statement and prior to the termination of this offering and any reports on Form 6-K subsequently submitted to the SEC or portions thereof that we specifically identify in such forms as being incorporated by reference into the registration statement of which this prospectus forms a part, shall be considered to be incorporated into this prospectus by reference and shall be considered a part of this prospectus from the date of filing or submission of such documents.

As you read the above documents, you may find inconsistencies in information from one document to another. If you find inconsistencies between the documents and this prospectus, you should rely on the statements made in the most recent document. All information appearing in this prospectus is qualified in its entirety by the information and financial statements, including the notes thereto, contained in the documents incorporated by reference herein.

We will provide to each person, including any beneficial owner, to whom this prospectus is delivered, a copy of these filings, at no cost, upon written or oral request to us at the following address:

Mereo BioPharma Group plc

1 Cavendish Place

4th Floor

London, W1G 0QF

United Kingdom

Tel: +44-333-023-7300

Attention: Investor Relations

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103,098,851 American Depositary Shares

Representing 515,494,255 Ordinary Shares

PRELIMINARY

PROSPECTUS

                , 2020

PART II—INFORMATION NOT REQUIRED IN THE PROSPECTUS

Item 8. Indemnification of directors and officers

The Articles of Association of Mereo BioPharma Group plc (“Mereo,” the “Company” or the “registrant”) provide that Mereo may indemnify the directors and other officers of Mereo in respect of any proceedings, whether civil or criminal, brought against them by reason of their being directors or officers of Mereo and to the fullest extent permitted by the Companies Act 2006 of the United Kingdom (“CA 2006”).

Generally, under CA 2006, any provision by which Mereo directly or indirectly provides an indemnity (to any extent) for a director of Mereo or of an “associated company” (i.e., a company that is a parent, subsidiary or sister company of Mereo) against any liability attaching to him or her in connection with any negligence, default, breach of duty or breach of trust in relation to the company of which he or she is a director is (subject to certain exceptions specified under CA 2006) void.

Mereo has entered into a deed of indemnity with each of its directors. Except as prohibited by applicable law, these deeds of indemnity may require Mereo, among other requirements, to indemnify its directors and executive officers for certain expenses, including attorneys’ fees, costs and expenses incurred by such directors and executive officers with the prior written consent of Mereo in any action or proceeding arising out of their service as a director or executive officer of Mereo, or one of its subsidiaries.

Mereo maintains directors’ and officers’ insurance coverage, which, subject to policy terms and limitations, is expected to include coverage to reimburse Mereo for amounts that it may be required or permitted by law to pay directors or officers of Mereo.

Insofar as indemnification for liabilities arising under the Securities Act of 1933, as amended (the “Securities Act”), may be permitted to directors, officers or persons controlling Mereo pursuant to the foregoing provisions, Mereo has been informed that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable.

Item 9. Exhibits and financial statements

(a) Exhibits. The exhibits to this registration statement are listed in the Exhibit Index to this registration statement and incorporated herein by reference.

(b) Financial Statement Schedules. Schedules have been omitted because the information required to be set forth therein is not applicable or is shown in our combined financial statements or the notes thereto.

Item 10. Undertakings

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EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form F-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in London, the United Kingdom on July 6, 2020.

KNOW ALL PERSONS BY THESE PRESENT, that each person whose signature appears below hereby constitutes and appoints Denise Scots-Knight, Ph.D., as his or her true and lawful attorney-in-fact and agent, with full power of substitution and resubstitution, for him or her and in his or her name, place and stead in any and all capacities, in connection with this registration statement, including to sign in the name and on behalf of the undersigned, this registration statement and any and all amendments thereto, including post-effective amendments and registrations filed pursuant to Rule 462 under the U.S. Securities Act of 1933, and to file the same, with all exhibits thereto, and other documents in connection therewith, with the U.S. Securities and Exchange Commission, granting unto such attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorney-in-fact and agent, or his or her substitute, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, as amended, this registration statement has been signed by the following persons on July 6, 2020 in the capacities indicated:

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SIGNATURE OF AUTHORIZED U.S. REPRESENTATIVE OF REGISTRANT

Pursuant to the requirements of the Securities Act of 1933, as amended, the undersigned, the duly authorized representative in the United States of Mereo BioPharma Group plc has signed this registration statement on July 6, 2020.

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Exhibit 5.1

Dear Sir/Madam

Registration Statement on Form F-3

We have acted for Mereo BioPharma Group plc, a public limited company incorporated under the laws of England and Wales (the “Company”), as its legal advisers in England in connection with the registration statement on Form F-3 (the “Registration Statement”) to be filed on or about 6 July 2020 by the Company with the U.S. Securities and Exchange Commission under the U.S. Securities Act of 1933, as amended (the “Securities Act”), and the rules and regulations promulgated thereunder (the “Rules”). The Registration Statement relates to the resale, by the selling shareholders identified in the Registration Statement, of up to an aggregate of 515,494,255 ordinary shares of £0.003 each in the Company (the “New Shares”), which New Shares may be exchanged for American Depository Shares (“ADS”) of the Company with each ADS representing 5 ordinary shares, consisting of (a) 89,144,623 ordinary shares issued to the selling shareholders pursuant to a securities purchase agreement dated 3 June 2020 (the “Securities Purchase Agreement”) by and among the Company and the investors listed on Exhibit A-1 thereto (the “Purchasers”), (b) 253,048,551 ordinary shares, being the number of ordinary shares issued as of the date hereof upon the conversion the convertible loan notes issued to the Purchasers pursuant to the Securities Purchase Agreement (the “Tranche 1 Notes”) and issuable upon the full conversion of the Tranche 1 Notes remaining outstanding as of the date hereof assuming the maximum amount of accrued interest for which additional ordinary shares are issuable upon conversion (including both accrued interest through the date hereof and accruing to the maturity date of the Tranche 1 Notes assuming such notes remain outstanding until their maturity date), (c) 161,048,366 ordinary shares issuable upon the full exercise of warrants issued to the Purchasers pursuant to the Securities Purchase Agreement (the “Warrants”) and (d) 12,252,715 ordinary shares issued to Boxer Capital, LLC in connection with a private placement in February 2020.

This is a legal communication, not a financial communication. Neither this nor any other communication from this firm is intended to be, or should be construed as, an invitation or inducement (direct or indirect) to any person to engage in investment activity.

Mayer Brown International LLP is a limited liability partnership (registered in England and Wales number OC303359) which is authorised and regulated by the Solicitors Regulation Authority. We operate in combination with other Mayer Brown entities with offices in the United States, Europe and Asia and are associated with Tauil & Chequer Advogados, a Brazilian law partnership.

We use the term “partner” to refer to a member of Mayer Brown International LLP, or an employee or consultant who is a lawyer with equivalent standing and qualifications and to a partner of or lawyer with equivalent status in another Mayer Brown entity. A list of the names of members of Mayer Brown International LLP and their respective professional qualifications may be inspected at our registered office, 201 Bishopsgate, London EC2M 3AF, England or on www.mayerbrown.com.

89,144,623 New Shares in aggregate were allotted to the Purchasers on 3 June 2020, 12,252,715 New Shares were allotted to Boxer Capital, LLC on or around 20 February 2020 and 125,061,475 New Shares in aggregate were allotted to the Purchasers on 30 June 2020 upon conversion of certain of the Tranche 1 Notes (such 226,458,813 New Shares together the “Initial Shares”, and the remaining New Shares the “Further Shares”).

The existing issued ordinary shares of the Company are admitted to trading, and application will be required to be made for the Further Shares to be admitted to trading, on the AIM market operated by London Stock Exchange plc (“AIM”).

In connection with the Registration Statement, we have been asked to provide an opinion on certain matters, as set out below.

For the purpose of this letter the Warrants and the Tranche 1 Notes are collectively referred to as the “Convertible Securities”, and New Shares and Convertible Securities are collectively referred to as the “Equity Securities”.

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and reviewed the information we received from our agents from the Searches (the “Search Results”).

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We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. In giving such consent, we do not admit that we are in the category of persons whose consent is required under section 7 of the Securities Act or the Rules.

Yours faithfully

/s/ Mayer Brown International LLP

Mayer Brown International LLP

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Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the reference to our firm under the caption “Experts” in the Registration Statement (Form F-3 ) and related Prospectus of Mereo Biopharma Group plc for the registration of 515,494,255 ordinary shares of its common stock and to the incorporation by reference therein of our report dated June 15, 2020, with respect to the consolidated financial statements of Mereo Biopharma Group plc included in its Annual Report (Form 20-F) for the year ended December 31, 2019, filed with the Securities and Exchange Commission.