California Attorney General Jerry Brown changed course on the state's new same-sex marriage ban Friday and urged the state Supreme Court to void Proposition 8.

In a dramatic reversal, Brown filed a legal brief saying the measure that amended the California Constitution to limit marriage to a man and a woman is itself unconstitutional because it deprives a minority group of a fundamental right. Earlier, Brown had said he would defend the ballot measure against legal challenges from gay marriage supporters.

"The amendment-initiative process does not encompass a power to abrogate fundamental constitutional rights without a compelling justification," he wrote. "Proposition 8 lacks such a justification."

Brown, who served as governor from 1975 to 1983, is considering seeking the office again in 2010. After California voters passed Proposition 8 on Nov. 4, Brown said he personally voted against it but would fight to uphold it as the state's top lawyer.

He submitted his brief in one of the three legal challenges to Proposition 8 brought by same-sex marriage supporters. The measure, a constitutional amendment that passed with 52 percent of the vote, overruled the Supreme Court decision last spring that briefly legalized gay marriage in the nation's most populous state.

Shannon Minter, legal director of the National Center for Lesbian Rights, called the attorney general's change of strategy "a major development."

"The fact that after looking at this he shifted his position and is really bucking convention by not defending Prop. 8 signals very clearly that this proposition can not be defended," Minter said.

The sponsors of Proposition 8 on Friday revealed for the first time that they would fight to undo the marriages of the estimated 18,000 same-sex couples who exchanged vows before voters banned gay marriage at the ballot box last month.

The Yes on 8 campaign filed a brief telling the court that because the new law holds that only marriages between a man and a woman are recognized or valid in California, the state can no longer recognize the existing same-sex unions.

"Proposition 8's brevity is matched by its clarity. There are no conditional clauses, exceptions, exemptions or exclusions," reads the brief co-written by Kenneth Starr, dean of Pepperdine University's law school and the former independent counsel who investigated President Bill Clinton.

Both Brown and gay rights groups maintain that the gay marriage ban may not be applied retroactively.

The Supreme Court could hear arguments in the litigation as soon as March. The measure's backers announced Friday that Starr, a former federal judge and U.S. solicitor general, had signed on as their lead counsel and would argue the cases.

The new brief provides a preview of how Proposition 8's supporters plan to defend the measure. It asserts that the Supreme Court lacks the authority or historical precedent to throw out Proposition 8.

"For this court to rule otherwise would be to tear asunder a lavish body of jurisprudence," the court papers state. "That body of decisional law commands judges — as servants of the people — to bow to the will of those whom they serve — even if the substantive result of what people have wrought in constitution-amending is deemed unenlightened."

Starr declined to discuss the arguments he and co-counsel Andrew Pugno advanced Friday. In an interview, Pugno said his clients' position on the same-sex marriages sanctioned in California between June 16 and Nov. 4 was not initiated by the Proposition 8 backers.

Rather, Pugno said the brief was a response to a question the court's seven justices posed to lawyers on both sides when they agreed to take up the challenge to Proposition 8 brought by gay rights advocates.

"The people passed Prop. 8," he said. "We are defending that."

Jennifer Pizer, a lawyer with the gay rights firm Lambda Legal who is helping represent gay and lesbian couples in the effort to undo the ballot measure, said Friday that vacating a marriage carries a host of legal repercussions and can not be accomplished so easily.

"This purported change exists in a landscape of a legal system that has rules, and those rules include the idea that changes in law affect people moving forward and do not apply looking backward to take away the vested rights people have," said Pizer, who recently married her longtime female partner.

The cases are Strauss v. Horton, S168047; City and County of San Francisco v. Horton, S168078; and Tyler v. State of California, S168066.