California Governor Arnold Schwarzenegger has confirmed he will veto a bill endorsing gay marriages.
The issue of gay marriage has taken hold across the US
State legislators voted on Tuesday to allow same-sex marriage in California, but the governor said the decision flew in the face of public opinion.
Five years ago Californians backed a proposition opposing the recognition of gay marriages in other states.
"We cannot have a system where the people vote and the legislature derails that vote," said the governor's office.
"Out of respect for the will of the people, the governor will veto," said Mr Schwarzenegger's press secretary, Margita Thompson.
Proposition 22, approved in a public vote in 2000, stated that "only marriage between a man and a woman is valid or recognised in California".
The bill approved by the California Assembly states that marriage is a civil contract between "two persons".
Gay rights activists accused the governor of playing to his Republican supporters, not the broad range of people who elected him to office.
"Clearly he's pandering to an extreme right wing, which was not how he got elected," said Geoff Kors, executive director of Equality California, one of the bill's sponsors.
"He sold us out."
More than 3,400 gay couples got married in San Francisco after the city's new mayor decided to defy state law and allow gay weddings in 2004.
But later in the year the state's Supreme Court ruled the mayor had exceeded his authority and nullified the unions.
In March this year a judge ruled that Californian state law had breached a constitutional right to equal treatment of all citizens, irrespective of sexuality.
The issue is now expected to go back to the Supreme Court.
Mr Schwarzenegger says he supports full legal protection for gay couples - but that the issue of gay marriage is best decided by the people or in the courts.