The attorney general of California urged the state Supreme Court on Friday to void a controversial same-sex marriage ban, saying the measure deprives gay couples of a fundamental right.

In a brief filed to the court, Jerry Brown said the measure that amended the California constitution to limit marriage to a man and a woman is unconstitutional.

The brief marks a change in position from Brown on the measure known as Proposition 8. After California voters passed the amendment on Nov. 4, Brown said he would fight to uphold the initiative in his role as attorney general, even though he personally voted against it.

Proposition 8 was the subject of a heated, costly battle ahead of election day and passed with 52 per cent of the vote. If it holds, the proposition will overrule a California Supreme Court decision last May that made marriage between two members of the same sex legal.

Gay-rights legal groups have filed a petition asking the Supreme Court to invalidate Proposition 8 on the grounds that voters did not have the authority to make such a dramatic change in state law without approval from the legislature.

On Nov. 19, the state Supreme Court decided to hear three legal challenges on the ban, but refused to allow gay couples to resume marrying until it rules.

Thirty states in the U.S. have banned gay marriage, although several permit civil unions or domestic partnerships that recognize some marriage rights.

The only two states that allow same-sex marriage are Massachusetts and Connecticut.

With files from the Associated Press