A federal appeals court on Thursday ruled that the Defense of Marriage Act is unconstitutional because it denies equal rights for legally married same-sex couples, making it likely that the Supreme Court will consider the politically divisive issue for the first time in its next term.

The unanimous decision by a panel of the U.S. Court of Appeals for the 1st Circuit in Boston was a big win for President Obama, who recently said he supported states allowing gay men and lesbians to marry.

His administration last year said it would no longer defend the 1996 law, which limits federal recognition of marriage to those between a man and a woman. The law thus denies a host of federal benefits, such as filing joint tax returns or receiving survivor benefits, to same-sex couples who were married in states that allow such unions.

The decision by a panel made up of judges nominated by presidents Ronald Reagan, George H.W. Bush and Bill Clinton did not address whether the Constitution provides gays and lesbians a fundamental right to marriage. It also did not address a part of DOMA that says states do not have to recognize same-sex marriages performed elsewhere.

Some gay rights activists have said that the limited question in the DOMA case made it a more attractive and incremental issue for an increasingly conservative Supreme Court than asking the justices to recognize a fundamental right of gays to marry.

The case presents only “the question of how the federal government treats people once they are married in their states,” said Mary L. Bonauto, who argued the case for a group called the Gay and Lesbian Advocates and Defenders (GLAD).

“We think this is a very solid decision to go before the Supreme Court.”

The decision by the Obama administration not to defend DOMA — a law that was signed but is now disavowed by Clinton — prompted an angry response from House Republican leaders. They hired Paul D. Clement, who was solicitor general in the administration of George W. Bush, to defend the law in court.

“We have always been clear we expect this matter ultimately to be decided by the Supreme Court, and that has not changed,” Clement said in a statement.

The federal judges said the same. The case “couples issues of equal protection and federalism” with the need to defer to Congress when it shows a rationale for passing a statute, wrote Circuit Judge Michael Boudin, the Bush appointee. There are precedents helpful to each side, he said.

“We have done our best to discern the direction of these precedents, but only the Supreme Court can finally decide this unique case,” he wrote. The most likely timetable is that the case could reach the justices in the fall, and if they decide to take it, it could be decided next year.

Boudin wrote that the congressional record on why DOMA was needed was sparse, and for 150 years, “the desire to maintain tradition would alone have been justification enough for almost any statute.”

But the Supreme Court, he said, requires Congress to speak with “special clarity” when it affects disadvantaged groups and when the action concerns issues traditionally left to the states, such as marriage.

While the congressional record on the act is “filled with encomia to heterosexual marriage,” the opinion states, Congress did not explain “how denying benefits to same-sex couples will reinforce heterosexual marriage.”

And the Supreme Court has ruled, the panel said, that “moral disapproval” is not adequate to justify discrimination based on sexual orientation.

Thirty-eight states adopt a prohibition on same-sex marriage in either law or their constitutions. Since DOMA was passed in 1996, same-sex marriage has become legal in eight states — Massachusetts, Connecticut, New York, Iowa, New Hampshire, Vermont, Maryland and Washington — and the District of Columbia. Maryland’s and Washington’s laws have not taken effect and may be challenged in referendums.

“One virtue of federalism is that it permits this diversity of governance based on local choice, but this applies as well to the states that have chosen to legalize same-sex marriage,” Boudin wrote.

“Under current Supreme Court authority, Congress’ denial of federal benefits to same-sex couples lawfully married in Massachusetts has not been adequately supported by any permissible federal interest.”

Conservative groups criticized the ruling. “Under this rationale, if just one state decided to accept polygamy, the federal government and perhaps other states would be forced to accept it, too,” said Dale Schowengerdt, legal counsel for the Alliance Defense Fund, which describes itself as a coalition of Christian lawyers.

Boudin’s decision, which was joined by Chief Circuit Judge Sandra L. Lynch and Circuit Judge Juan R. Torruella, was the first time an appeals court has agreed with a challenge to DOMA.

But it is part of a string of legal decisions that gay-rights activists have won on same-sex marriage. Two U.S. District Court judges in California have found the same section of DOMA considered by the Boston court to be unconstitutional, and those cases will soon be considered by the U.S. Court of Appeals for the 9th Circuit in San Francisco.

Additionally, the 9th Circuit is deciding whether the full court should review a three-judge panel’s decision in February that struck down California’s Proposition 8, a voter-approved constitutional amendment that defines marriage as only between a man and a woman.

But voters have rejected same-sex marriage when it came before them in referendum, the latest example being North Carolina in May.

There were two cases in the 1st Circuit’s decision, Massachusetts v. HHS and Gill v. Office of Personnel Management. Gill was brought by seven same-sex couples married in Massachusetts and three surviving spouses of such marriages who were denied federal benefits and recognition. The decision said about 100,000 couples would be affected.

“How thrilling it is for us that the court believes in protecting our rights,” said one of the plaintiffs, Bette Jo Green, 70. She said she has been with her partner, Jo Ann Whitehead, for more than 31 years, and the two married in 2004.