The New York Times reports that Federal Attorney-General Eric Holder Jr sent a letter to Congress earlier in the week saying that the Justice Department would be taking the position in court that the Act, which bars federal recognition of same-sex marriage, should be struck down as a breach of the right of same-sex couples to have equal protection under the law.

"The president and I have concluded that classifications based on sexual orientation warrant heightened scrutiny," wrote Holder.

The move comes after last year's repeal of the military's ban on gays and lesbians, and the change of policy on marriage law will be likely to intensify the long-running political and ideological debate over same-sex marriage.

"It's a monumentally important decision," said Tobias Wolff, a law professor at the University of Pennsylvania who advised Obama on gay rights issues.

"The Justice Department and the president have taken the position on behalf of the United States government that discrimination against gay and lesbian people in all cases is presumptively unconstitutional.

"It's the first time the United States Government has ever embraced that position, and if the courts agree, it will help to eradicate all of the various forms of discrimination that gay and lesbian people suffer around the country."