Conservative Cabinet members, junior ministers and party enforcers are among around 180 MPs poised to oppose or abstain in a vote on gay marriage, it has been claimed.
Prime Minister David Cameron has also been sent a letter, signed by 25 chairmen or former chairmen of Conservative Party associations, warning that the policy will cause "significant damage" at the ballot box, according to the Sunday Telegraph.
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 earlier this week it would not include marriage tax breaks in next month's budget - something that would have been seen as a concession to disgruntled traditionalists.
Mr Cameron views the introduction of same-sex marriage as the "Conservative Party delivering the promise it made".
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."
Mr Cameron is giving Conservative MPs a free vote on the Marriage (Same Sex Couples) Bill.
Environment Secretary Owen Paterson and Welsh Secretary David Jones are expected to vote against the plans while Defence Secretary Philip Hammond will vote against or abstain and Work and Pensions Secretary Iain Duncan Smith is poised to abstain, according to Sunday Telegraph.
Culture minister Ed Vaizey insisted the vote would not tear the Tory party apart and claimed internal divisions over the issue were "good-natured". He told Murnaghan on Sky News: "Is this going to tear the Tory party apart? No, I don't think it will. I think there is a difference of opinion among colleagues but it is a civilised debate."
Former children's minister Tim Loughton agreed that it would not tear the party apart but warned the Bill was "full of pitfalls" and would set MP against MP. He told Murnaghan: "There's quite a lot of things that were in our manifesto which made it to the coalition agreement which we have yet to deliver, and yet gay marriage is something which we had no green paper, no white paper, no manifesto commitment of any party, it wasn't in the coalition agreement, and all of a sudden it is taking huge priority, it is going to take up a lot of parliamentary time and is going set MP against MP, and we don't need it."