Around 180 Conservative MPs are reportedly poised to oppose or abstain in a vote on gay marriage to be held on Tuesday.
MPs will vote on the proposals, which will also allow civil partners to convert their partnership to a marriage and enable married people to change their legal gender without having to end their union, for the first time when the Bill has its second reading on Tuesday.
Backbenchers have made no secret of their opposition to the move and were left even more angered when the Tory leadership made clear it would not include marriage tax breaks in next month's Budget - something that would have been seen as a concession to disgruntled traditionalists.
Twenty five chairmen or former chairmen of Conservative Party associations have written a letter to the Prime Minister, warning him that the policy will cause "significant damage" at the ballot box.
The letter calls for a decision to be postponed until after the 2015 general election.
It closes by saying: "Resignations from the Party are beginning to multiply and we fear that, if enacted, this Bill will lead to significant damage to the Conservative Party in the run up to the 2015 election."
David Cameron views the introduction of same-sex marriage as the "Conservative Party delivering the promise it made".
But Geoffrey Vero from Conservative Grassroots told Sky News: "It's very dangerous to scratch the core beliefs of many people and this is a matter which should be taken much more slowly and debated across the country."
Minister for culture Ed Vaizey said he did not think the issue would tear the Tory party apart and told Sky News he thought it was a "civilised debate".
Health Secretary Jeremy Hunt, a big supporter of reforms, said: "Every year thousands of people choose to marry in a church rather than a registry office because they believe marriage is sacred. Religious freedom is not just for heterosexuals - we should not deny anyone the right to make a lifelong commitment to another person in front of God if that is what they believe and that is what their church allows."
Education Secretary Michael Gove has also thrown his support behind the proposed legalisation of same-sex marriage and promised teachers would not be disciplined for refusing to promote it.
Concerns have been raised ahead of the Commons vote that thousands of teachers could face the sack because they object to the new law.
But Mr Gove made clear that while teachers would have to explain that it is legal, they would not be required to actively promote it.
Writing in the Mail on Sunday, Mr Gove explained his stance, saying it was wrong to say to gay men and women that their love is less legitimate.
He said: "It's wrong to say that because of how you love and who you love, you are not entitled to the same rights as others. It's wrong because inequality is wrong."
He continued: "Marriage is not undermined by extending it to gay people - it is reinforced by including everyone equally."
Environment Secretary Owen Paterson and Welsh Secretary David Jones are expected to vote against, while Defence Secretary Philip Hammond will vote against or abstain and Work and Pensions Secretary Iain Duncan Smith is poised to abstain, according to The Sunday Telegraph.
Tony and Barrie Drewitt-Barlow are gay Christians who would like to marry in their local church where their five children were baptised. Tony said: "It's just about fairness, it's just about being the same as everybody else. We're not asking for more than anyone else, just the same."