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 25, 2017 (January 25, 2017)

Former name or former address, if changed since last report: None

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:

On January 25, 2017, WGL Holdings, Inc. (“WGL”) issued a news release announcing the execution of an Agreement and Plan of Merger, pursuant to which, among other things and subject to the satisfaction or waiver of specified conditions, AltaGas Ltd. agreed to acquire WGL in an all cash transaction. A copy of the news release announcing the signing of the Merger Agreement is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

(d) Exhibits.

Additional Information and Where to Find It

This communication may be deemed to be solicitation material in respect of the proposed merger transaction. WGL Holdings, Inc. (“WGL”) intends to file with the U.S. Securities and Exchange Commission (the “SEC”) and mail to its shareholders a proxy statement in connection with the proposed merger transaction. THE INVESTORS AND SECURITY HOLDERS OF WGL ARE URGED TO READ THE PROXY STATEMENT AND ANY OTHER RELEVANT DOCUMENTS WHEN THEY BECOME AVAILABLE, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION about AltaGas, Ltd. (“AltaGas”), WGL and the proposed merger transaction. Investors and security holders will be able to obtain these materials (when they are available) and other documents filed with the SEC free of charge at the SEC’s website, www.sec.gov. In addition, a copy of WGL’s proxy statement (when it becomes available) may be obtained free of charge upon request by contacting WGL Holdings, Inc., Leslie T. Thornton, Corporate Secretary, 101 Constitution Avenue N.W., Washington, District of Columbia, 20080. WGL’s filings with the SEC are also available on WGL’s website at: http://wglholdings.com/sec.cfm. Investors and security holders may also read and copy any reports, statements and other information filed by WGL with the SEC, at the SEC public reference room at 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 or visit the SEC’s website for further information on its public reference room.

Participants in the Solicitation

AltaGas, WGL and certain of their respective directors, executive officers and other persons may be deemed to be participants in the solicitation of proxies in respect of the proposed merger transaction. Information regarding AltaGas’ directors and executive officers is available in AltaGas’ Management Information Circular, filed on March 17, 2016 (in English and French) with the Canadian Securities Administrators (the “CSA”) and in AltaGas’ Annual Information Form, filed on March 23, 2016 (in English) and March 24, 2016 (in French) with the CSA, each of which are available at: www.sedar.com. Information regarding WGL’s directors and executive officers is available in WGL’s proxy statement filed with the SEC on December 23, 2016 in connection with its 2017 annual meeting of shareholders, and its Annual Report on Form 10-K for the fiscal year ended September 30, 2016, each of which may be obtained from the sources indicated in Additional Information and Where to Find It. Other information regarding persons who may be deemed participants in the proxy solicitation and a description of their direct and indirect interests (which may be different than those of WGL’s investors and security holders), by security holdings or otherwise, will be contained in the proxy statement and other relevant materials filed or to be filed with the SEC when they become available.

*            *             *

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrants have duly caused this report to be signed on their behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX