WASHINGTON, (AFP) -- US President Barack Obama threw his weight behind gay marriage on Thursday when his administration filed a brief with the Supreme Court opposing California's bid to ban same-sex unions.

The court is to examine the issue at the end of March, when it will study the constitutionality of California's "Proposition 8," a measure approved by referendum in 2008 which outlawed gay marriage in the state.

Obama's Department of Justice filed a brief to the court in support of moves to have the measure overturned, arguing that it violates the 14th Amendment to the Constitution which guarantees citizens equal rights.

"In our filing today in Hollingsworth v. Perry, the government seeks to vindicate the defining constitutional ideal of equal treatment under the law," Attorney General Eric Holder said in a statement.

"Throughout history, we have seen the unjust consequences of decisions and policies rooted in discrimination," he 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 gay rights groups which challenged the California law welcomed the White House support, which 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, president of the Human Rights Campaign.

"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."