"This sets up an all-or-nothing showdown at the United States Supreme Court," said Brown, who asked for contributions to help fund a possible Supreme Court challenge to the lower court's ruling.

The passage of Proposition 8 inserted language into the state constitution that marriage was between one man and one woman, overriding the California Supreme Court, which struck down a pair of laws in the summer of 2008 that had restricted marriage to one man and one woman.

Auxiliary Bishop Gerald Wilkerson of Los Angeles, president of the California Catholic Conference, expressed disappointment in the ruling but also confidence that it would be reversed.

"We are disappointed by the ruling today by a panel of the 9th Circuit that would invalidate the action taken by the people of California affirming that marriage unites a woman and a man and any children from their union," he said in a Feb. 7 statement.

"However, given the issues involved and the nature of the legal process, it's always been clear that this case would very likely be decided by the U.S. Supreme Court. Marriage between one man and one woman has been — and always will be — the most basic building block of the family and of our society.

"In the end, through sound legal reasoning, we believe the court will see this as well and uphold the will of the voters as expressed in Proposition 8. We continue to pray for that positive outcome."

An estimated 18,000 same-sex marriages had been performed in California before 52 per cent of the state's voters in 2008 approved Proposition 8, which banned same-sex marriages in the state.

ProtectMarriage, which put Proposition 8 on the ballot and fought in court to uphold it, can appeal the decision, either to the full 9th Circuit or directly to the U.S. Supreme Court. In the meantime, the appeals panel said no same-sex marriages can be performed.

The appellate court majority said it did not answer the question of "whether under the Constitution same-sex couples may ever be denied the right to marry," calling it "an important and highly controversial question."

"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," said the majority decision, written by Judge Stephen Reinhardt. "There was no such reason that Proposition 8 could have been enacted.

"Proposition 8 served no purpose, and had no effect, other than to lessen the status and human dignity of gays and lesbians in California," it added.

All Proposition 8 did, the court said, was to strip same-sex couples of "the right to obtain and use the designation of 'marriage' to describe their relationships. Nothing more, nothing less. Proposition 8 therefore could not have been enacted to advance California's interests in child-rearing or responsible procreation, for it had no effect on the rights of same-sex couples to raise children or on the procreative practices of other couples. Nor did Proposition 8 have any effect on religious freedom or on parents' rights to control their children's education."

The 9th Circuit issued its opinion in an appeal filed by supporters of traditional marriage after now-retired U.S. District Court Judge Vaughn Walker ruled the voter-approved initiative was unconstitutional under the due process and equal protection clauses of the 14th Amendment to the U.S. Constitution.

The group challenged Walker's standing in the case, because the judge, who is gay, did not disclose his sexual orientation or reveal that he had a long-time romantic partner until after he had retired from the court. The 9th Circuit panel said it found no evidence Walker was biased.

The three-judge panel's decision is on hold at least until the deadline passes for ProtectMarriage to file an appeal, meaning no same-sex marriages would be conducted in California in the near future.

Currently, seven states and the District of Columbia permit same-sex marriage. Among the states considering a same-sex marriage bill is Washington, which falls within the jurisdiction of the 9th Circuit. The governor, Christine Gregoire, a Catholic, has said she will push for its passage in the state legislature.