For the nation’s estimated 80,000 legally married same-sex couples, the Obama adminstration’s decision to stop defending the federal law that bans the recognition of gay marriage will have little immediate effect.

Lawyers and gay rights activists said Thursday that the administration’s announcement was one step — albeit an important one — in a battle over the law’s constitutionality that is likely to play out in the federal courts over the next several years.

The Justice Department on Wednesday said it would no longer go to court to oppose challenges to the 1996 Defense of Marriage Act, which defines marriage as between a man and a woman and denies marriage-based federal benefits to same-sex married couples. The administration said it no longer considers the law constitutional.

The decision drew outrage from Republicans and applause from gay rights activists, who have won a series of political victories. But underlying the euphoria was a recognition that nothing had changed for same-sex married couples who say the law discriminates against them, and that the U.S. Supreme Court is likely to have the last word.

“There’s going to be at least a couple of years of litigation over this, and sooner or later the Supreme Court is going to have to weigh in,’’ said Darren Rosenblum, a professor at Pace Law School in New York and an expert on gay and lesbian rights.

While legal experts differed on whether the law is constitutional, they said the Justice Department’s change of position will have a major impact on appellate courts that consider the issue. As the government’s lawyer, the department is generally given significent weight in constitutional cases.

“This is the executive branch saying to the judicial branch that we believe the law is unconstitional. Courts will pay a lot of attention to that,’’ said Jon Davidson, legal director for Lambda Legal, which fights for gay and lesbian rights.

The Defense of Marriage Act states that for federal purposes, “the word ‘marriage’ means only a legal union between one man and one woman as husband and wife.’’ Gay couples can legally marry in five states and the District. But the federal law means that legally wed same-sex couples are denied an array of benefits available to heterosexual couples. Those include the ability to file joint federal tax returns, to be exempt from certain federal taxes and to have a spouse who is an immigrant legally remain in the United States.

Six challenges to the Defense of Marriage Act are working their way through federal courts, including in Massachusetts, where a judge in July struck down the law. The Justice Department had appealed that decision.

Officials said the department will now advise courts that it will no longer defend the law. Members of Congress can mount a defense, and opponents of same-sex marriage have called on House Speaker John A. Boehner (R-Ohio) to intervene.

A spokesman for Boehner declined to comment Thursday.

But even if Congress doesn’t officially step in, lawyers said they expect that the Massachussetts case and others will still be decided by appellate courts that can weigh briefs filed by same-sex marriage opponents.

The new legal battle began Thursday in California, where a judge in one of the cases challenging the federal marriage law ordered the government to explain how its change of position affects the case. The suit was filed on behalf of a federal employee who was denied health benefits for her legal spouse. The Justice Department has until March 7 to respond.