(Reuters) – Two groups of American International Group Incshareholders won class-action status from a federal judge on Mondayin a $25 billion lawsuit by former Chief Executive Maurice“Hank” Greenberg over alleged losses caused by the U.S.government's bailout of the insurer.

U.S. Court of Federal Claims Judge Thomas Wheeler alsoappointed Greenberg's lawyer, David Boies, of Boies, Schiller& Flexner LLP, as lead counsel for the classes.

Greenberg's Starr International Co, once AIG's largestshareholder with a 12 percent stake, sued theUnited States in2011 over what eventually became a $182.3 billion bailout for theNew York-based insurer.

It said that by taking a 79.9 percent AIG stake andthen conducting a reverse stock split without letting existingshareholders vote, the government conducted an illegal taking thatviolated the 5th Amendment of the U.S. Constitution.

Citing Boies' estimate that “tens of thousands” of shareholdersmight be affected, Wheeler said “class certification is by far themost efficient method of adjudicating these claims.”

He distinguished the case from the U.S. Supreme Court's2011 rejection of class status for more than 1million Wal-Mart Stores Inc workers alleging gender bias,saying the AIG claims are “based on the same exactgovernment action” rather than “literally millions” of separateactions.

One class includes AIG shareholders as of Sept. 22,2008, when a credit agreement awarding the 79.9 percent stake tookeffect. The other class includes shareholders as of June 30, 2009who were denied a chance to vote on the reverse split.

U.S. Department of Justice spokesman CharlesMiller declined to comment.

AIG decided on Jan. 9 not to join Greenberg's lawsuit, amidanger from Congress and voters at the prospect that itmight sue the same entity that rescued it from collapse.

Greenberg is separately appealing the Nov. 19, 2012 dismissal ofa related lawsuit in Manhattan federal court againstthe Federal Reserve Bank of New York.

On March 1, AIG bought back warrants from the TreasuryDepartment, eliminating the government's last financial interest inthe insurer.

The case is Starr International Co. v. U.S., U.S. Courtof Federal Claims, No. 11-00779.