Conservative Cabinet members, junior ministers and party enforcers are among around 180 MPs poised to oppose or abstain in a vote on gay marriage.
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.
SENIOR TORIES WHO COULD OPPOSE BILL
SENIOR TORIES WHO COULD ABSTAIN
Iain Duncan Smith
The news comes after a poll for ComRes, on behalf of the anti-gay marriage Coalition For Marriage, found 40,000 teachers said they would refuse to teach pupils about gay marriage, and risk the sack.
The survey out showed 74,000 — one in six — “wouldn’t be happy about it.”
Colin Hart of the Coalition for Marriage warned: “Thousands of teachers face the real prospect of being disciplined or sacked.”
But Education Secretary Michael Gove said that teachers would not be forced to teach anything against their beliefs.
Writing in the Mail on Sunday, he said: "Some teachers are concerned that equal marriage legislation may require them to teach views which go against their beliefs and open them to threats - either in the workplace or the courts - from those who hold diverging views.
"I want to reassure those teachers today because so many of our best, and wisest, teachers are people of deep religious faith.
"Some of the schools I most admire are faith schools and their success is inextricably bound up with their religious ethos.
"If I thought any legislation, however well-intentioned, would make life more difficult for great teachers and great schools I wouldn't support it."
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.
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."
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.
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.
Culture minister Ed Vaizey insisted the vote would not tear the Tory party apart and claimed internal divisions over the issue were "good-natured".
"We'll see what happens in the vote on Tuesday, various numbers have been bandied about but what I would say is that it is good-natured division," 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."
Vaizey insisted the Bill was not diverting government from rebuilding the economy, welfare reform and pushing through major infrastructure projects.
"Equal civil marriage is not distracting government from these important tasks," he said.
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.
Foreign Secretary William Hague said he became a supporter of gay marriage "over the last couple of years".
He said he considered whether it was right in principle, if there was enough public support and if there were protections for people who did not agree with it.
"I think as times have changed, civil partnerships came in, within a remarkably short period of time those things become accepted," he told BBC 1's Sunday Politics. "I think the same will happen with this."