LONDON - Britain's parliament voted heavily in favor of legalising gay marriage on Tuesday, but Prime Minister David Cameron's authority in his own party took a blow as his Conservatives split in two over the measure he had championed.

In the first of several votes required for its passage, the lower house of parliament backed the legislation by 400-175, but more than half of Cameron's 303 lawmakers voted against or abstained, signalling deep unease with it and his leadership.

Elizabeth Maddison (L) kisses her civil partner Hannah Pearson after proposing marriage to her in front of Parliament in London Feb 5, 2013. British lawmakers on Tuesday backed legalising gay marriage in the first of several votes on the issue after a debate which split Prime Minister David Cameron's ruling Conservative party in two. [Photo/Agencies]

During a debate that lasted more than six hours, many Conservative MPs denounced the legislation, saying it was morally wrong, not a public priority, and unnecessarily divisive, threatening a corrosive legacy of bitterness.

Conservative lawmaker Gerald Howarth told parliament that the government had no mandate to push through a "massive social and cultural change".

"This is not evolution, it's revolution," added Edward Leigh, another Conservative member of parliament, saying marriage was "by its nature a heterosexual union".

Although the vote went Cameron's way, many analysts believe he will now have to address a deep seam of discontent running through his party.

He made a last minute televised statement ahead of the vote, arguing gay marriage would make society stronger.

"I'm a big believer in marriage. It helps people to commit to each other, and I think that's why gay people should be able to get married too," he said.

He later hailed the result of the vote as "a step forward for our country".

Cameron is trying to perform a tricky balancing act: to reconcile his desire to show his party is progressive, with the views of many in it who are uncomfortable with such a reform.

Amid talk of a possible leadership challenge to Cameron, many Conservative lawmakers say the prime minister is sacrificing core party values on the altar of populism.

"He hasn't got a lot of political capital left in the bank," Stewart Jackson, a Conservative MP who opposes the gay marriage bill, told Reuters before the vote. "He has to deliver some authentic Conservative policies very soon."

Such talk is rife among some Conservative lawmakers and follows a spate of articles in the British press in which a handful of MPs raised the possibility of ousting Cameron, a prospect most commentators regard as far-fetched before the next election in 2015.

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