David Cameron got his way as gay marriage legislation cleared its first hurdle in the Commons - but saw his party split down the middle.
After the Prime Minister made a last ditch appeal for support, the House backed the proposals by a big margin of 400 to 175.
However, with Labour and Liberal Democrats strongly in favour, it was clear that scores of Tories had taken advantage of the free vote to register their opposition.
The Labour Whips office suggested that 139 Tories had voted against the Bill, with 132 in favour. Dozens more did not vote.
The result followed more than six hours of stormy debate on the Marriage (Same Sex Couples) Bill.
Culture Secretary Maria Miller said it would make Britain "a fairer place to live", and insisted religious organisations which did not want to conduct gay marriages had protection.
But Tory MPs lined up to condemn the measures - including the chairman of the backbench 1922 Committee Graham Brady, who said he had "serious misgivings" over assurances on religious freedom.
Former defence minister Sir Gerald Howarth said that the Government had no mandate for such a "massive social and cultural change", which was not mentioned in the Conservative manifesto for the 2010 election.
Speaking in Downing Street less than two hours before the crunch vote, Mr Cameron accepted that there were "strong views on both sides of the argument".
But he said: "I am a strong believer in marriage. It helps people commit to each other and I think it is right that gay people should be able to get married too."