A federal court ruled Tuesday that California's voter-approved ban on same-sex marriage was unconstitutional.

A three-judge panel of the 9th U.S. Circuit Court of Appeals upheld a ruling issued in August 2010 that Proposition 8 violated the 14th Amendment of the Constitution, which guarantees equal protection under the law. In that ruling, U.S. District Judge Vaughn Walker wrote that the ban "fails to advance any rational basis in singling out gay men and lesbians for denial of a marriage license."

On Tuesday, the appellate judges ruled, "By using their initiative power to target a minority group and withdraw a right that it possessed, without legitimate reason for doing so, the People of California violated the Equal Protection Clause. We hold Proposition 8 to be unconstitional on this ground."

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Protect Marriage, proponents of Prop. 8, had argued that restricting marriage to heterosexuals would promote responsible child-rearing by married biological parents. They also argued that Walker, who is gay and who was in a long-term relationship at the time he heard the case, should have recused himself, because he had a conflict of interest because he may have wanted to marry.

Another federal judge, James Ware, found in June 2011 that Walker had no obligation to refuse to take the case. The sponsors of Prop. 8 appealed that decision.

The 9th Circuit panel on Tuesday unanimously agreed with Ware.

But Tuesday's ruling on the issue of same-sex marriage was narrow and not unanimous. The judges ruled 2-1 that because California's domestic partnership law already provided same-sex couples all of the rights of opposite-sex couples, regardless of their marital status, "Proposition 8 serves no purpose, and has no effect, other than to lessen the status of gays and lesbians in California, and to officially classify their relationships and families as inferior to those of opposite-sex couples. The Constitution simply doesn't allow for 'laws of this sort.'"

Judge N. Randy Smith was the sole dissenter. He wrote that he was "not convinced that Proposition 8 is not rationally related to a legitimate government interest."

The ruling will not take effect for at least 14 days and is likely to be appealed. Prop. 8 supporters have three options for filing such an appeal: they could ask the three-judge panel that issued Tuesday's ruling to reconsider; they could ask for a hearing with 11 judges from the 9th Circuit Court of Appeals; or they could appeal directly to the U.S. Supreme Court.

Opponents and supporters of same-sex marriage believe that the Supreme Court will eventually weigh in on the issue.

Andy Pugno, the general counsel for Protect Marriage, said his organization had not yet decided whether to appeal directly to the Supreme Court.

But he did say, “We will immediately appeal this misguided decision that disregards the will of more than 7 million Californians who voted to restore marriage as the unique union of only a man and woman. We are confident that the rights of California voters will finally win out."

Mayor Ed Lee, who, in a statement, said he "celebrated" the 9th Circuit Court's ruling, vowed that San Francisco "will take this fight all the way to the nation’s highest court, if necessary."

Protect Marriage did not immediately respond to requests for comment.

Gavin Newsom, who, as mayor of San Francisco in 2004, ordered the city clerk to issue marriage licenses to same-sex couples, tweeted Tuesday, "We won!!"

Sen. Dianne Feinstein tweeted, "Today’s landmark #Prop8 decision is a win FOR equality and AGAINST discrimination. A big step toward marriage equality in Calif."

And Gov. Jerry Brown, who had declined to defend Prop. 8, said in a statement, "The court has rendered a powerful affirmation of the right of same-sex couples to marry. I applaud the wisdom and courage of this decision."

The panel that issued Tuesday's ruling included Judge Stephen Reinhardt, who was appointed by President Jimmy Carter; Judge Michael Daly Hawkins, who was appointed by President Bill Clinton; and Smith, who was appointed by President George W. Bush, as was Walker.

The plaintiffs in the case are the American Foundation for Equal Rights and two same-sex couples. They are represented by Theodore Olson and David Boies. Olson represented George W. Bush and Boies represented Al Gore in Bush v. Gore, the Supreme Court case that helped bring an end to the 2000 presidential election.

Voters passed Prop. 8 in Nov. 2008, six months after the state Supreme Court struck down Proposition 22, which prohibited the state from performing same-sex marriages or recognizing those performed in other states. In its ruling, the court called marriage "a basic civil right" for all Californians, "whether gay or heterosexual."

From June 2008 through November 2008, approximately 18,000 same-sex couples were married in California. But with the passage of Prop. 8 that year and the subsequent court battles, the legal status of those marriages remains unclear.

Same-sex marriage is legal in six states — New York, Connecticut, Iowa, Massachusetts, New Hampshire and Vermont — and the District of Columbia. Washington state's Legislature may approve a measure to legalize such marriages later this week.