Megachurch Pastor Rick Warren has given his backing to a major campaign to overturn same-sex "marriage" in California.

The Golden State heads to polling stations on November 4 to vote on Proposition 8, a proposed amendment to California's state constitution that would see a ban on same-sex unions, with potentially major repercussions for states across the US.

Warren, pastor of Saddleback Church and author of "The Purpose Driven Life", said in his News and Views email to church members, "For 5,000 years, every culture and every religion - not just Christianity - has defined marriage as a contract between men and women.

"There is no reason to change the universal, historical definition of marriage to appease 2% of our population."

He added that the California State Supreme Court "threw out the will of the people" when it decided to overturn Proposition 22, which declares marriage to be a union between one man and one woman. The proposition was upheld by 61 per cent of voters in 2000 but thrown out in May this year when the court deemed it discriminatory towards same-sex couples and therefore unconstitutional.

"I never support a candidate but on moral issues I come out very clear," he said.

Rick Warren was on the list of Time magazine's 100 most influential people in the world and 15 world leaders who matter most. His endorsement of traditional marriage will be seen as a major boost for the Californian churches that are fasting and praying that the amendment will get the "yes" vote.

In a civil forum in August, Warren challenged US presidential candidates, Democrat Barack Obama and Republican John McCain on their views of marriage. Both senators affirmed that marriage was a union between one man and one woman.