David Cameron is to offer Conservative MPs a free vote on the Government's plans to legalise gay marriage amid growing discontent in his ranks.
Critics questioned the Prime Minister's commitment to the reform after it emerged that he will not whip his party to support the move in the division lobbies.
Mr Cameron is personally supportive of the reform and Downing Street stressed again this week the Government's determination to get it on to the statute book before the end of the parliament.
Downing Street initially refused to say whether there would be a free vote on the legislation, saying it was "premature to start talking about whipping" when the issue was still under consultation and there was not yet a Bill.
But Leader of the Commons Sir George Young later confirmed there would be a free vote.
"Along with other issues that involve matters of conscience, it seems to me perfectly proper that this should be subject to a free vote on this side of the House and that is what we plan to do," he told the Commons.
His Labour shadow, Angela Eagle, said the policy would now only become law with her party's support.
"Lacking the courage of the Prime Minister's convictions and threatened with a growing revolt inside the Cabinet, they have decided to grant those opposed to equal marriage a free vote," she said.
"This means that the Government's flagship policy on equal rights will only become law with Labour support."
Gay rights campaigner Peter Tatchell said: "Since the Government is committed to legalising same-sex marriage it is somewhat surprising that it has decided to opt for a free vote.
"If it is official Government policy, surely Government MPs should be required to vote for it?
"Relegating it to a free vote suggests that David Cameron does not regard marriage equality as an important principle and policy."
This week a Conservative Cabinet minister declared his opposition to same-sex marriage in the most high-level challenge to Mr Cameron so far on the issue.
Northern Ireland Secretary Owen Paterson made clear his hostility to the change in a letter to a constituent.
Defence Secretary Philip Hammond said recently that it was "not a priority" for the Government after the Liberal Democrat policy was blamed by some for poor local election results.
Defence minister Gerald Howarth is also opposed to same-sex marriage.
Tory critics of the plan see it, like House of Lords reform, as a Liberal Democrat policy that is a distraction from the bigger challenges facing the Government.
But Desmond Swayne, Mr Cameron's parliamentary private secretary, has recorded a video message for the cross-party Out4Marriage campaign backing the plans.
He said: "I'm married, I enjoy being married, it's a huge blessing, and therefore I want that blessing to be extended to everyone.
"I've come at this issue of equal marriage principally because I am a Christian.
"I believe that the promises of the Gospel are unconfined: they're for everyone, and the sacraments that follow from that should be available to everyone. That's why I'm Out4Marriage."
Home Secretary Theresa May also joined the Out4Marriage campaign today.
She said there were "strong views on both sides of this argument" and she would listen to all of them.
But she went on: "Marriage binds us together, it brings stability, I think marriage makes us stronger. But I believe also in commitment and in fidelity in marriage, I think these are good things and we should enable them to flourish.
"That's why I believe if two people care for each other, if they love each other, if they want to commit to each other and spend the rest of their lives together then they should be able to get married and that marriage should be for everyone, and that's why I'm coming Out4Marriage."