A U.S. appeals court Thursday declared that the federal Defense of Marriage Act unconstitutionally denies federal benefits to married gay couples, a groundbreaking ruling all but certain to wind up before the U.S. Supreme Court.

The unanimous ruling said the 1996 law that defines marriage as a union between a man and a woman discriminates against gay couples because it doesn't give them the same rights and privileges as heterosexual couples.

The three-judge panel of the 1st U.S. Circuit Court of Appeals wasn't asked to address whether gay couples have a constitutional right to marry.

The panel also didn't rule on the law's other politically combustible provision, which said states without same-sex marriage cannot be forced to recognize gay unions performed in states where it's legal.

The law was passed at a time when it appeared Hawaii would legalize gay marriage. Since then, many states have instituted their own bans on gay marriage, while eight states have approved it: Massachusetts, Connecticut, New York, Iowa, New Hampshire, Vermont, Maryland and Washington state, plus the District of Columbia.

Last year, President Barack Obama announced the U.S. Department of Justice would no longer defend the constitutionality of the law.

The appeals court, the first federal appeals panel to deem the benefits section of the law unconstitutional, agreed with a lower-court judge who ruled in 2010 that the Defense of Marriage Act is unconstitutional because it interferes with the right of a state to define marriage and denies married gay couples federal benefits given to heterosexual married couples, including the ability to file joint tax returns.

The 1st Circuit said its ruling wouldn't be enforced until the U.S. Supreme Court decides the case, meaning that same-sex married couples will not be eligible to receive the economic benefits denied by the law until the high court rules.

That's because the ruling only applies to states within the circuit, including Massachusetts, Rhode Island, Maine, New Hampshire and Puerto Rico. Only the Supreme Court has the final say in deciding whether a law passed by Congress is unconstitutional.

Although most Americans live in states where the law says marriage can only be the union of a man and a woman, the power to define marriage had always been left to the individual states before Congress passed the Defense of Marriage Act, the appeals court said in its ruling.

"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," Judge Michael Boudin wrote for the court. "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."

During arguments before the court last month, a lawyer for gay married couples said the law amounts to "across-the-board disrespect." The couples argued that the power to define and regulate marriage had been left to the states for more than 200 years.

An attorney defending the law argued that Congress had a rational basis for passing it in 1996, when opponents worried that states would be forced to recognize gay marriages performed elsewhere. The group said Congress wanted to preserve a traditional and uniform definition of marriage and has the power to define terms used to federal statutes to distribute federal benefits.

Gay & Lesbian Advocates & Defenders, the legal group that brought one of the lawsuits on behalf of gay married couples, said the court agreed with the couples that it is unconstitutional because it takes one group of legally married people and treats them as "a different class" by making them ineligible for benefits given to other married couples.

"We've been working on this issue for so many years, and for the court to acknowledge that yes, same-sex couples are legally married, just as any other couple, is fantastic and extraordinary," said Lee Swislow, GLAD's executive director.