In what could be a historic advance for gay rights in the United States, Barack Obama’s administration has announced that it will not defend the Defense of Marriage Act in two specific cases because it defines a marriage as only being between a man and a woman.

The decision was made by Obama himself, because he believes that the law is unconstitutional. However, a spokesman for the White House said that it is not a reflection of a change of Obama’s own position on gay marriage.

The president is still “grappling” with his personal views on the matter, however, he has consistently described the Defense of Marriage act as “unnecessary” and “unfair”.

The 1996 federal law denies couples who have legally married in US states which allow it the same federal protection that hetrosexual couples enjoy. An US citizen can not sponsor their non-US citizen same sex spouse in the same way that straight couples can.

The decision not to defend the law is as a result of two cases brought against the US government. They were filed in Connecticut and New York by gay rights groups argued that the law was unconstitutional. The plaintiffs were five married same-sex couples and a widower who have all been denied federal rights and protections because they are married to a person of the same sex.

Mr Obama’s decision not to defend the anti-gay legislation mirrors that of former governor of California, Arnold Schwarzenegger not to defend a case brought against the state of California’s Proposition 8, a voter initiated ban on gay marriage. But others may be able to defend claims in the place of the administration, including Congressional leaders.

In a statement, Attorney General Eric Holder said “much of the legal landscape has changed in the 15 years since Congress passed [the Defense of Marriage Act]‘.

He said there were no longer any “reasonable arguments” to deny gay married couples the same rights as straight couples.

He added: “There is, regrettably, a significant history of purposeful discrimination against gay and lesbian people, by governmental as well as private entities, based on prejudice and stereotypes that continue to have ramifications today.”

The attorney general also pointed out that the Supreme Court has already ruled that laws that criminalise homosexual behaviour are unconstitutional.

The law remains on the statute books however, until Congress repeals it or a court strikes it down. But Ted Olson, the former solicitor general to president George W Bush said that the decision is “certainly going to be persuasive in federal courts that even the government, who had a responsibility to defend the statutes if it could find a basis for doing so, felt that a ‘heightened scrutiny’ does apply.” Mr Olsen is currently leading the case against Proposition 8, the ban on gay marriage in California.

“This is a monumental decision,” Joe Solmonese of the Human Rights Campaign (HRC). “Congressional leaders must not waste another taxpayer dollar defending this patently unconstitutional law. The federal government has no business picking and choosing which legal marriages they want to recognize.”