SAN FRANCISCO — A federal appeals court on Tuesday declared California's same-sex marriage ban to be unconstitutional, putting the bitterly contested, voter-approved law on track for likely consideration by the U.S. Supreme Court.

A three-judge panel of the 9th U.S. Circuit Court of Appeals ruled 2-1 that a lower court judge correctly interpreted the U.S. Constitution when he declared in 2010 that Proposition 8 was a violation of the civil rights of gays and lesbians.

The passage of the ban followed the most expensive campaign on a social issue in U.S. history.

It was unclear when gay marriages might resume in California. Lawyers for Proposition 8 sponsors and for the two couples who successfully sued to overturn the ban have said they would consider appealing to a larger panel of the court and then the U.S. Supreme Court if they did not receive a favourable ruling.

"Although the Constitution permits communities to enact most laws they believe to be desirable, it requires that there be at least a legitimate reason for the passage of a law that treats different classes of people differently. There was no such reason that Proposition 8 could have been enacted," the ruling states.

The court crafted a narrow decision that applies only to California, even though the court has jurisdiction in nine western states.

"Whether under the Constitution same-sex couples may ever be denied the right to marry, a right that has long been enjoyed by opposite-sex couples, is an important and highly controversial question," the court said. "We need not and do not answer the broader question in this case."

The panel also said there was no evidence that former Chief U.S. Judge Vaughn Walker was biased and should have disclosed before he issued his lower-court decision that he was gay and in a long-term relationship with another man. Walker publicly revealed he was gay after he retired.

Proposition 8 backers had asked the 9th Circuit to set aside Walker's ruling on constitutional grounds and because of the judge's personal life. It was the first instance of an American jurist's sexual orientation being cited as grounds for overturning a court decision.

Walker's successor as the chief federal judge in Northern California, James Ware, rejected the claims about Walker's personal life.

California voters passed Proposition 8 with 52 per cent of the vote in November 2008, five months after the state Supreme Court legalized same-sex marriage by striking down a pair of laws that had limited marriage to a man and a woman.

The ballot measure inserted the one man-one woman provision into the California Constitution, thereby overruling the court's decision. It was the first such ban to take away marriage rights from same-sex couples after they had already secured them.

The Williams Institute on Sexual Orientation and the Law, a think-tank based at the University of California, Los Angeles, has estimated that 18,000 couples married during the four-month window before Proposition 8 took effect. The California Supreme Court upheld those marriages but ruled that voters had properly enacted the law.

With same-sex marriages unlikely to resume in California any time soon, Love Honor Cherish, a gay rights group based in Los Angeles, plans to start gathering signatures for a November ballot initiative asking voters to repeal Proposition 8.