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.
It names whips Stephen Crabb, David Evennett, Robert Goodwill, Mark Lancaster, Nicky Morgan and John Randall as likely to oppose of abstain as well as junior ministers Mike Penning, John Hayes and Jeremy Wright. Mr Wright said: "I will listen to the arguments in favour on Tuesday, but I am not persuaded by what I have heard so far."
Culture Secretary Maria Miller insists the plans will ensure "equal and fair" treatment for same-sex couples, while protecting religious institutions that do not want to perform ceremonies.
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