BOSTON (AP) - 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 court 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.

"For me, it's more just about having equality and not having a system of first- and second-class marriages," said plaintiff Jonathan Knight, 32, a financial associate at Harvard Medical School who married Marlin Nabors in 2006.

"I think we can do better, as a country, than that," Knight said.

Knight said DOMA costs the couple an extra $1,000 a year because they cannot file a joint federal tax return.

Opponents of gay marriage blasted the decision.

"This ruling that a state can mandate to the federal government the definition of marriage for the sake of receiving federal benefits, we find really bizarre, rather arrogant, if I may say so," said Kris Mineau, president of the Massachusetts Family Institute.

Since DOMA was passed in 1996, many states have instituted their own bans on gay marriage, while eight states have approved it, including Massachusetts, Connecticut, New York, Iowa, New Hampshire, Vermont, Maryland, Washington state and the District of Columbia. Maryland and Washington's laws are not yet in effect and may be subject to referendums.

Last year, President Barack Obama announced the U.S. Department of Justice would no longer defend the constitutionality of the law. After that, House Speaker John Boehner convened the Bipartisan Legal Advisory Group to defend it. The legal group argued the case before the appeals court.

White House spokesman Jay Carney said the appeals court ruling is "in concert with the president's views." Obama, who once opposed gay marriage, declared his unequivocal personal support on May 9.

Carney wouldn't say whether the government would actively seek to have DOMA overturned if the case goes before the Supreme Court.

"I can't predict what the next steps will be in handling cases of this nature," Carney said.

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 - Massachusetts, Rhode Island, Maine and 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 still is that 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 DOMA, 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 before Congress passed DOMA.

Paul Clement, a Washington, D.C., attorney who defended the law on behalf of the Bipartisan Legal Advisory Group, 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.

Clement did not immediately return a message left Thursday. The legal group could ask for the case to be reheard by the full 1st Circuit, which typically sits six judges, or could ask the U.S. Supreme Court to take on the case.

Gay & Lesbian Advocates & Defenders, the Boston-based legal group that brought one of the lawsuits on behalf of seven gay married couples and three widowers, said the law 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.

Two of the three judges who decided the case Thursday were Republican appointees, while the other was a Democratic appointee. Boudin was appointed by President George H.W. Bush, while Judge Juan Torruella was appointed by President Ronald Reagan. Chief Judge Sandra Lynch is an appointee of President Bill Clinton.

In California, two federal judges have found this year that the Defense of Marriage Act violates the due process rights of legally married same-sex couples.

In the most recent case, a judge found the law unconstitutional because it denies long-term health insurance benefits to legal spouses of state employees and retirees. The judge also said a section of the federal tax code that makes the domestic partners of state workers ineligible for long-term care insurance violates the civil rights of people in gay and lesbian relationships.