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 to reach 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 and Supreme Court precedents when he declared in 2010 that Proposition 8 — a response to an earlier state court decision that legalized gay marriage — was a violation of the civil rights of gays and lesbians.

However, the appeals court said gay marriages cannot resume in the state until the deadline passes for Proposition 8 sponsors to appeal to a larger panel of the 9th Circuit. If such an appeal is filed, the panel’s ruling would remain on hold until it’s resolved.

“Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California, and to officially reclassify their relationships and families as inferior to those of opposite-sex couples,” states the opinion written by Judge Stephen Reinhardt, one of the court’s most liberal judges.

More than 50 people who gathered outside the federal courthouse in downtown San Francisco greeted the ruling with cheers. They held signs and waved rainbow flags.

Backers of Proposition 8 said they would ask the Supreme Court to overturn the 9th Circuit ruling, which came more than a year after the appeals court panel heard arguments in the case.

“We are not surprised that this Hollywood-orchestrated attack on marriage — tried in San Francisco — turned out this way. But we are confident that the expressed will of the American people in favor of marriage will be upheld at the Supreme Court,” said Brian Raum, senior counsel for the Alliance Defense Fund, a Christian legal aid group based in Arizona that helped defend Proposition 8.

Supporters of gay marriage hailed the ruling. American Foundation for Equal Rights President Chad Griffin, who formed the group along with director Rob Reiner to wage the court fight against Proposition 8, called the panel’s decision “a historic victory.”

“The message it sends to young LGBT people, not only here in California but across the country, (is) that you can’t strip away a fundamental right,” Griffin said.

The appeals panel crafted a narrow decision that applies only to California, even though the court has jurisdiction in nine western states. California is the only one of those states where the ability for gays to marry was granted then rescinded.

“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 decision that he was gay and in a long-term relationship with another man.

Proposition 8 backers had asked the 9th Circuit to set aside Walker’s ruling on constitutional grounds and because of the thorny issue 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 publicly revealed he was gay after he retired. Supporters of the gay marriage ban argued that he had been obliged to previously reveal if he wanted to marry his partner — like the gay couples who sued to overturn the ban.

The 9th Circuit held a hearing on the conflict-of-interest question in December. In its ruling Tuesday, the panel majority said it was unreasonable to presume a judge cannot apply the law impartially just because he is a member of the minority group at issue in a case.

“To hold otherwise would demonstrate a lack of respect for the integrity of our federal courts,” the opinion said.

Reihardt, who was appointed to the appeals court by President Jimmy Carter, was joined in the majority opinion by Judge Michael Hawkins, an appointee of President Bill Clinton.

Judge Randy Smith, who was appointed by President George W.  Bush, dissented, saying he disagreed that Proposition 8 served no purpose other than to treat gays and lesbians as second-class citizens.

California voters passed Proposition 8 with 52 percent 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 and its passage followed the most expensive campaign on a social issue in the nation’s history.

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 tied the knot 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.

Gov. Jerry Brown, in his previous role as attorney general, and former Gov. Arnold Schwarzenegger, refused to defend Proposition 8 in court and left it to the ballot measure’s sponsors to appeal Walker’s decision to the 9th Circuit.

Brown issued a statement Tuesday saying, “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.”

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 gather signatures for a November ballot initiative asking voters to repeal Proposition 8.