UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported) January 14, 2014

Tower Group International, Ltd.

(Exact name of registrant as specified in its charter)

Bermuda Commercial Bank Building

19 Par-La-Ville Road

Hamilton, HM 11, Bermuda

(Address of principal executive office)

(441) 279-6610

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Item 8.01. Other Events

Tower Group International, Ltd. (“Tower”) has received as of January 14, 2014, all required insurance regulatory approvals for an agreement Tower reached with AmTrust Financial Services, Inc. (“AmTrust”) and National General Holdings Corporation (“NGHC”), pursuant to which a subsidiary of AmTrust and a subsidiary of NGHC would provide 100% quote share reinsurance and a cut through endorsement to cover all eligible new and renewal commercial and personal lines business, respectively, and, at their option, losses incurred on or after January 1, 2014 on not less than 60% of the in-force business. Tower received confirmation on January 16, 2014 from AmTrust and NGIC that they would exercise such option to reinsure on a cut through basis losses incurred on or after January 1, 2014 under in-force policies with respect to (1) in the case of AmTrust, a significant majority of Tower’s unearned premium reserves as of December 31, 2013 with respect to its ongoing commercial lines business, and (2) in the case of NGIC, 100% of Tower’s unearned premium reserves as of December 31, 2013 with respect to its personal lines segment business.

Additional Information and Where to Find It

This communication is not a solicitation of a proxy from any shareholder of Tower. In connection with the Agreement and Plan of Merger (the “Merger Agreement”) entered into by and among Tower, ACP Re Ltd. (“ACP”) and a wholly-owned subsidiary of ACP (“Merger Sub”), pursuant to which, subject to the satisfaction or waiver of the conditions therein, it is expected that Merger Sub will merge with and into Tower (the “Merger”), Tower intends to file a proxy statement with the United States Securities and Exchange Commission (“SEC”). Investors and shareholders are urged to read the proxy statement and other relevant materials filed with the SEC when they become available because they will contain important information about Tower, ACP Re and the proposed transaction. The proxy statement and other relevant materials (when they become available), and any other documents filed by Tower or ACP Re with the SEC, may be obtained free of charge at the SEC’s website at www.sec.gov. In addition, investors and shareholders may obtain free copies of the documents filed by Tower with the SEC by directing a written request to “Investor Relations,” Tower Group International, Ltd., Bermuda Commercial Bank Building, 2nd Floor, 19 Par-la-Ville Road, Hamilton, HM 11, Bermuda, or by email to Bernard Kilkelly, Managing Vice President, Investor Relations (bkilkelly@twrgrp.com).

Participants in the Solicitation

The directors, executive officers and other members of management and employees of Tower may be deemed participants in the solicitation of proxies from its stockholders in favor of the transactions. Information concerning persons who may be considered participants in the solicitation of Tower’s stockholders under the rules of the SEC is set forth in public filings filed by Tower with the SEC and will be set forth in the proxy statement/prospectus when it is filed with the SEC. Information concerning Tower’s participants in the solicitation is contained in Tower’s Proxy Statement on Schedule 14A, filed with the SEC on March 25, 2013.

Cautionary Statement Regarding Forward–Looking Statements

The Private Securities Litigation Reform Act of 1995 provides a “safe harbor” for forward-looking statements. This Current Report on Form 8-K and any other written or oral statements made by or on behalf of Tower may include forward-looking statements that reflect Tower’s current views with respect to future events and financial performance. All statements other than statements of historical fact included in this Current Report on Form 8-K are forward-looking statements. Forward-looking statements can generally be identified by the use

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of forward-looking terminology such as “may,” “will,” “plan,” “expect,” “project,” “intend,” “estimate,” “anticipate,” “believe” and “continue” or their negative or variations or similar terminology. All forward-looking statements address matters that involve risks and uncertainties. Accordingly, there are or will be important factors that could cause the actual results of Tower to differ materially from those indicated in these statements. Please refer to Tower’s filings with the SEC, including among others Tower’s Annual Report on Form 10-K for the year ended December 31, 2012 and subsequent filings on Forms 10-K/A, 10-Q and 10-Q/A, for a description of the important factors that could cause the actual results of Tower to differ materially from those indicated in these statements. Forward-looking statements speak only as of the date on which they are made, and Tower undertakes no obligation to update publicly or revise any forward-looking statement, whether as a result of new information, future developments or otherwise.

Risks that could adversely affect the proposed Merger include, but are not limited to, the following:

The following important factors are among those that could affect the actual outcome of other future events:

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Additional risk factors that may cause outcomes that differ from our expectations or projections are described in various documents filed by Tower with the Securities and Exchange Commission, such as current reports on Form 8–K, and regular reports on Forms 10–K and 10–Q, particularly in “Item 1A, Risk Factors.”

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SIGNATURES

Pursuant to the requirement of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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