Divisions over plans to legalise gay marriage were exposed as 70 MPs voted for a "wrecking" amendment designed to scupper the Government's controversial Marriage (Same Sex Couples) Bill by allowing straight couples to take up civil partnerships.
Plans to allow same-sex marriage survived a Conservative backlash after a fraught debate in the House of Commons.
An amendment which supporters warned could derail the legislation altogether was easily defeated after the Government reached an agreement with Labour.
The Marriage (Same Sex Couples) Bill now looks set to be approved by MPs tomorrow - but is likely to meet stiff resistance in the House of Lords.
A bid by a Tory opponent of gay marriage to extend civil partnerships to heterosexual couples attracted 70 supporters in a free vote - far fewer than the 100-plus anticipated by some critics.
Ex-minister Tim Loughton insisted it was not a "wrecking amendment" and was a move backed by the public and many advocates of same-sex marriage.But the change - which Culture Secretary Maria Miller said would impose significant delays and costs - was defeated by a majority of 305.
The Prime Minister's spokesman has hinted the Government will accept a Labour amendment tonight on extending civil partnerships - a move that should ensure the Same Sex Marriages Bill becomes law.
The Culture Secretary Maria Miller had argued for a lengthy consultation over five years about whether civil partnerships should be extended to included heterosexual couples. Labour today put forward a Manuscript Note - a form of amendment - demanding an immediate consultation.
The Manuscript Note is entirely consistent with the Secretary of State's amendment, which is that there should be a full consultation within five years.
– David Cameron's spokesman
The implication is that the Government and Labour can reach agreement, to ensure the Same Sex Marriage bill goes through.
Just 127 Conservatives endorsed the proposals at second reading in the Commons.
The smooth progress of the bill at third reading is less assured, though, thanks to Conservative MP's Tim Loughton's proposed amendment to the bill to allow heterosexuals to have the same right to civil partnerships as gay couples.
His suggested measure is attracting support from supporters of equal marriage as well as those who want to stop the legislation.
Labour's frontbench team is set to support Mr Loughton's amendment, giving it a real prospect of success in a free vote.