US President Barack Obama's administration threw its weight behind gay marriage on Thursday, urging the Supreme Court to strike down California's ban on same-sex unions.
The court is set to examine the issue on March 26, when it will study the constitutionality of California's Proposition 8, a measure approved by a 2008 referendum that outlawed gay marriage in the most populous US state.
In a separate brief to the court concerning another case, the administration has asked justices to declare the 1996 Defense of Marriage Act -- a law that defines marriage as a union between a man and a woman -- unconstitutional.
The Justice Department filed the latest brief in support of moves to have the California measure overturned, arguing it violates the 14th Amendment to the Constitution that guarantees citizens equal rights.
"Throughout history, we have seen the unjust consequences of decisions and policies rooted in discrimination," Attorney General Eric Holder warned.
"The issues before the Supreme Court in this case... are not just important to the tens of thousands of Americans who are being denied equal benefits and rights under our laws, but to our nation as a whole."
The filing by Solicitor General Donald Verrilli is more narrowly focused on the California ban, and does not seek a ruling that would apply nationwide.
The administration's brief noted that seven other states -- Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island -- have measures that grant same-sex couples rights similar to those of married couples, while restricting marriage to heterosexual unions.
Those states would be affected by the California ruling.
The California law "provides to same-sex couples registered as domestic partners all the legal incidents of marriage, but it nonetheless denies them the designation of marriage, Verrilli wrote.
Therefore, "the exclusion of gay and lesbian couples from marriage does not substantially further any important governmental interest," he added.
Gay marriage opponents have seized upon the same similarities to claim there is no discrimination, saying California provides essentially the same rights and obligations of marriage to same-sex domestic partners.
The Family Research Council, a conservative Christian group promoting the traditional family unit, highlighted what it called the Obama administration's "hypocrisy."
"President Obama, who was against same-sex 'marriage' before he was for it, and his administration, which said the Defense of Marriage Act was constitutional before they said it was unconstitutional, has now flip-flopped again on the issue of same-sex 'marriage,' putting allegiance to extreme liberal social policies ahead of constitutional principle," FRC president Tony Perkins said in a statement.
"For the Obama administration to now challenge Proposition 8 -- 'federalizing' what is a settled matter under California state law -- is an act of brazen hypocrisy."
Nine states and the US capital Washington currently allow gay marriage. The states include Connecticut, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New York, Vermont and Washington.
The federal government is not a party in the case, but its friend-of-the-court brief marked a victory for gay rights groups challenging the California law.
The White House's support had been expected since Obama shifted his stance on the same-sex marriage question before his re-election last year.
"President Obama and the solicitor general have taken another historic step forward consistent with the great civil rights battles of our nation's history," said Chad Griffin, head of the Human Rights Campaign gay advocacy group.
"The president has turned the inspirational words of his second inaugural address into concrete action by urging our nation's highest court to put an end to discrimination against loving, committed gay and lesbian couples."
Last month, Obama made the first-ever direct reference to gay rights in a US president's inaugural address, saying: "Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law.
"For if we are truly created equal, then surely the love we commit to one another must be equal as well."