The Obama administration on Wednesday delivered what has been described by the San Francisco Chronicle to be a "rare and important legal policy reversal", declaring that a law prohibiting federal recognition of same-sex marriages is unconstitutional and discriminatory and that Department of Justice lawyers will no longer defend it in court cases across the country.

The Defense of Marriage Act, which was passed in 1996, defines marriage as only between a man and a woman, and prohibits the federal government from granting benefits to same-sex couples, such as breaks for estate taxes and Social Security payments to spouses.

The administration will still enforce the law as the responsibility for defending the marriage act now falls to Congress, which has the authority to appoint a special counsel to pursue the issue.

In a statement issued by the US Department of Justice that notified Congress of President Barack Obama's decision, Attorney General Eric Holder said: "After careful consideration, including a review of my recommendation, the president has concluded that given a number of factors, including a documented history of discrimination, classifications based on sexual orientation should be subject to a more heightened standard of scrutiny.

"Much of the legal landscape has changed in the 15 years since Congress passed DOMA. The Supreme Court has ruled that laws criminalising homosexual conduct are unconstitutional. Congress has repealed the military’s Don’t Ask, Don’t Tell policy. Several lower courts have ruled DOMA itself to be unconstitutional. Section 3 of DOMA will continue to remain in effect unless Congress repeals it or there is a final judicial finding that strikes it down, and the President has informed me that the Executive Branch will continue to enforce the law. But while both the wisdom and the legality of Section 3 of DOMA will continue to be the subject of both extensive litigation and public debate, this Administration will no longer assert its constitutionality in court."