The ruling was the latest setback to efforts by the gay community to challenge laws that restrict the institution of marriage to heterosexual couples.

The court said the mayor, Gavin Newsom, and city officials violated the law when they issued the certificates, since legislation and a state voter-approved measure defined marriage as a union between a man and woman.

The judges decided by a 5-2 vote to nullify the 3,995 gay marriages performed in the city between February 12 and March 11, when the court halted the weddings. Their legality, Justice Joyce Kennard wrote, must wait until courts resolve the constitutionality of state laws that restrict marriages to opposite-sex couples.

The court ordered officials in San Francisco to "undo" their previous "unauthorised" actions, correct official records and notify the newlyweds that their marriages were void.

The ruling disappointed gay rights advocates who had hoped the weddings would remain valid while the broader arguments of the constitutionality of excluding same-sex couples from the institution of marriage were thrashed out. Gay couples faced the reversal with a mixture of defiance and dismay.

"No court can take away the love we share," said John Lewis, who married his partner, Stuart Gaffney, in the city six months ago.

Phyllis Lyon, 79, who mar ried Del Martin, 83, said: "After being together for more than 50 years, it is a terrible blow to have the rights and protections of marriage taken from us. At our age, we do not have the luxury of time."

The issue of gay marriage has become a thorny one in the US after two key legal rulings last year. The first, by the US supreme court, struck down sodomy laws, effectively removing the last legal obstacle to gay relationships.

The second, in Massachusetts, made gay marriage legal in that state alone, prompting President George Bush and other Republicans to call for an amendment to the US constitution that would limit mar riage to heterosexuals. Many states have already amended their statutes to do that.

While neither of the Democratic party's White House challengers, John Kerry and John Edwards, supports gay marriage, both are opposed to a constitutional amendment. The right of same-sex couples to have their relationships legally recognised is expected to become a central social issue of the presidential campaign.

The ruling "is important, but it will not resolve whether same-sex couples have the right to marry and be treated equally under our state constitution", Jennifer Pizer, a lawyer with the Lambda Legal Defence and Education Fund, a gay rights legal aid group, told the Associated Press.

The issue of whether the mayor exceeded his authority had already been conceded by most gay activists. Kate Kendell, the director of the National Centre for Lesbian Rights, said it was clear Mr Newsom would be overruled.

"We think that's unfortunate and wrong, but the more human question is what to do with marriage licences that belong to couples in relationships and with families."