A federal appeals court Thursday declared that the 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.

In its unanimous decision, the three-judge panel of the 1st U.S. Circuit Court of Appeals in Boston said the 1996 law that defines marriage as a union between a man and a woman deprives gay couples of the rights and privileges granted to heterosexual couples.

The court didn’t rule on the law’s more politically combustible provision, which said states without same-sex marriage cannot be forced to recognize gay unions performed in states where it’s legal. It also wasn’t asked to address whether gay couples have a constitutional right to marry.

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, led by Massachusetts in 2004.

The court, the first federal appeals panel to deem the benefits section of the law unconstitutional, agreed with a lower level judge who ruled in 2010 that the law 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 DOMA 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.