The coalition's controversial gay marriage legislation returns to the Commons later with Tory MPs and activists deeply split over the issue.The move has been championed by David Cameron but he has faced Conservative opposition at all levels from the grassroots to the Cabinet.
A group of more than 30 current and former local party chairmen warned that the plans would drive Tory voters to the UK Independence Party and make a Conservative election victory in 2015 impossible.
Bob Woollard, chairman of the Conservative Grassroots group which organised the protest letter to the Prime Minister, said: "Same sex marriage is really a tipping point, a bellwether issue if you like - people have just said 'I've had enough, I'm off, I will never vote Conservative again'.
"Scores and scores and scores of people that we have all spoken to, probably hundreds of thousands of people, have said: 'I've had enough, that's it now, we can't cope with this so-called modernisation agenda.
"We are not voting Conservative again until this bill is scrapped, defeated in the House of Lords, kicked into the long grass or until there's a change of leadership'."
But a rival letter, signed by more than 100 Tory activists, called for Conservative MPs to "deal with the Bill then move on together as a party".
The Marriage (Same Sex Couples) Bill will be debated over two days, with its third reading - the final hurdle in the Commons - on Tuesday.
Tory former minister Tim Loughton will attempt to amend the legislation to allow heterosexuals to have the same right to civil partnerships as gay couples.
He highlighted a ComRes survey of 159 MPs from across the parties which found 73% agreed that civil partnerships should be extended to heterosexuals "in the interests of equality" if gay marriage is legalised.
Mr Loughton said: "Far from being a 'wrecking measure' some of the strongest support for my amendment to extend civil partnerships comes from the biggest supporters of same sex marriage in the Labour and Lib Dem parties.
"If the Government think it is right to extend marriage to everyone then it has to be right to extend civil partnerships to everyone too.
"This can only be good for improving stability for many more of the near three million opposite sex couples who currently choose to cohabit but are in no formally recognised relationship."
But the Conservative Equalities Minister, Helen Grant, said there was "no significant demand from heterosexual couples for this to happen" and called the amendment a "distraction" that would delay the legislation.
She welcomed the free vote for MPs over the Bill, dismissing the grassroots party rebellion.
"None of us would be here without our loyal, hardworking grassroots people in our various constituencies. Of course these issues are a concern but we knew with something as emotive and polarised as gay marriage there would be differences. That's why the PM has given a free vote," she told Sky News.
Mr Loughton is also supporting amendments which will give registrars the option to opt-out of holding same-sex unions on faith grounds and also protect teachers if they refuse to promote gay marriage.
Labour's equalities team, led by shadow home secretary Yvette Cooper, is set to support Mr Loughton's amendment on heterosexual civil partnerships.
Ms Cooper warned on Sky News' Murnaghan programme that it would be a "real problem" if the legislation "gets lost in the vortex of the Tory infighting".
With free votes for MPs, Mr Loughton's amendment could pass despite the opposition of ministers.
Sky News Deputy Political Editor Joey Jones said the issue of gay marriage was "very uncomfortable" for the Conservative party, which was "divided" over the matter.
"It is difficult because the leadership have set out a line which is in support of gay marriage yet Mr Cameron has some Cabinet colleagues that are voting against or complaining that too much time and energy is being spent on this when they think there are other more important like the economy," he explained.
Culture Secretary Maria Miller has tabled rival amendments to allow a review on extending civil partnerships to heterosexual couples, but only five years after gay marriage has been introduced.
It would pave the way for civil partnerships to be extended, or, if demand has plummeted, scrapped altogether.
Deputy Prime Minister Nick Clegg said it was Lib Dem policy that civil partnerships should be available to heterosexual couples but said he would not back amendments that would derail the Bill.
Speaking at a Nacro event in London, he said: "The bottom line is I will do whatever I judge is best to safeguard the bill and to make sure it doesn't become hijacked by those whose ulterior motive is actually to discredit or to derail the legislation."
He added: "In principle this is something we have long supported but I don't want to lose the wood for the trees. I want this bill to be passed and for the bill to do what it says on the tin which is to provide equal rights to marriage for same sex couples and that will be my overriding objective." A Government source said Mr Loughton's amendment would cost taxpayers £4bn as a result of extra pension liabilities and could delay the introduction of gay marriages by up to two years.
The source added that it "undermines marriage by creating a two-tiered system".
If the Bill clears the Commons it will head to the Lords, where fierce opposition to the proposals is expected.