Gay marriage laws are expected to clear their latest hurdle in the Commons tonight, after Labour agreed to back it in a series of votes.
David Cameron has faced down his backbenchers to press ahead with his plan for same-sex weddings, despite warnings the party is haemorrhaging grassroots support.
It came after former Tory donor Lord Ashcroft warned the party risks being plunged into a 'spiral of irrelevance'.
David Cameron is facing a Commons defeat tonight on gay marriage that could cost the Exchequer £4billion
The Prime Minister had been facing a Commons defeat tonight on gay marriage, amid warnings rebel demands to allow heterosexual couples to have civil partnerships could cost the Exchequer £4billion.
Mr Cameron is also under growing pressure after backing party co-chairman Lord Feldman against claims he called Eurosceptic activists 'mad, swivel-eyed loons'.
Lord Ashcroft, a former Tory party treasurer called for an end to the the rows over Europe, gay marriage and 'loongate'.
Writing on the ConservativeHome website, the Tory peer said: 'This sort of thing - the habit of distracting ourselves from what matters - has got to stop.
'The Conservatives were elected, albeit in insufficient numbers, to do a job. We hope to be elected in two years' time in rather greater numbers to implement a Tory manifesto in full.
'But we don't look as if either of those things is our priority. We need to pull ourselves out of what threatens to become a spiral of irrelevance.'
More... A chance encounter in the Blue Boar and a torrent of threats from No.10... where that 'swivel-eye loons' comment came from (and what followed it) 'Swivel eye' showdown: Furious Tory grandees will quiz PM's friend on claims he called party faithful 'swivel-eyed loons' MELISSA KITE: Cameron's chums DO despise the grass rootsAt least 100 Tory MPs are expected to back an amendment allowing heterosexual couples to have civil partnerships too.
But today Labour announced it would not back the rebels, and tabled their own amendment to allow for a public consultation on opposite-sex civil partnerships.
Shadow home secretary Yvette Cooper said Labour was offering to save the bill from being 'sucked into the vortex of Tory infighting'.
'Labour votes have got this bill through so far and Labour votes will get this through the next couple of days, when the Government is deeply divided and fighting itself,' Ms Cooper told BBC Radio 4's World at One.
'We are trying to find a sensible way through which allows us to make that progress and make sure this bill is not derailed either by the Government or by Tory backbenchers.'
During early debate in the Commons today, Tory MP Edward Leigh said: 'It seems to many of us if you dare to disagree with the new orthodoxy that gay marriage is the best thing since sliced bread, you are somehow breaking a new social taboo, you are doing something in your workplace, particularly in the public sector, that you should not be doing.'
He called for people who disagree with gay marriage to be given protection under the Equality Act 2010.
Sources said the Prime Minister had been ‘very concerned’ about losing the vote, which he believes would undermine the institution of marriage.
Mr Cameron's official spokesman said: 'The Equal Marriage Bill is about the extension of marriage.
'There have been concerns and issues raised in the House with regard to civil partnerships and that is why the Government has proposed a formal review of civil partnerships.
'The Prime Minister is a big believer in marriage and that is why he thinks gay people should be able to get married.'
Culture Secretary Maria Miller attempted to head off the vote last week by offering to hold a review of civil partnerships five years after gay marriage has been legalisedAsked whether the Bill will be dropped if the 'wrecking' amendment is passed this evening, the spokesman said: 'The Government has its legislative agenda and it is getting on with it.'
Grassroots Tories warned of a ‘crisis of conservatism’, saying the Prime Minister was badly out of touch with the party membership over gay marriage.
Opening up civil partnerships to all would be a far bigger move than gay marriage itself, which affects only relatively small numbers.
The amendment could delay implementation of the entire legislative package for two years – meaning the first gay weddings would be delayed until after the next election.
It would also open up the tax advantages of civil partnerships to 2.8million cohabiting couples – with costly implications for a number of government departments.
Increased spouse’s rights to public sector pensions alone could land the taxpayer with a bill of £3-£4billion. Inheritance tax receipts could be significantly reduced.
Rebel Tory MPs believe the Treasury could even demand the entire the Marriage (Same Sex Couples) Bill is pulled.
‘There is a sense we could be in trouble on this,’ said a Government source last night.
MPs will have a free vote on the issue today and Labour indicated the entire Shadow Cabinet and most MPs are expected to back extending civil partnerships to heterosexual couples as a matter of principle.
Lib Dem policy has long called for equality on civil partnerships and sources said many of the party’s MPs will back the amendment.
Culture Secretary Maria Miller attempted to head off the vote last week by offering to hold a review of civil partnerships five years after gay marriage has been legalised.
But supporters of the change say this does not go far enough.
Today Mrs Miller warned rebels against any action which would delay the introduction of gay marriage.
'I think it would make a fundamental difference to the passage of the bill because what the amendment is trying to import in a very complicated concept which actually took more than 200 clauses to explain in the original civil partnership bill to a bill which is all about extending marriage,' she told BBC Radio 4's Today programme.
'It introduces complexity which would not only I believe delay the passage of the bill through Parliament but it would also put forward a number of quite fundamental policy issues which need considerable thought and consultation, let alone any delays around implementation.
'I want to be seeing the marriages undertaken under this new bill as early as next summer.'
Opening up civil partnerships to all would be a far bigger move than gay marriage itself, which affects only relatively small numbersLabour last night said ministers had provided no evidence to justify their claims that the Bill will be delayed or that the amendment will cost lots of money.
A party source said: ‘It would be outrageous of the Government to use this issue as a reason to delay or kill the Bill. Is that what is going on?’
More than 150 Tory MPs are expected to defy the Prime Minister on a separate amendment, which would mean that teachers would be exempt from prosecution if they taught gay marriage was wrong, and would provide immunity for registrars who refused to conduct gay ceremonies.
But the amendment is not supported by the Labour and Lib Dem leadership and is likely to be rejected.
Senior Tory activists warned yesterday that gay marriage was making it ‘virtually impossible’ for the party to win the next election. In a letter to the Prime Minister, more than 30 present and former local party chairmen warned Mr Cameron’s backing for a change in the law had led to voters switching their support to UKIP.
The letter to Mr Cameron was organised by the Grassroots Conservative group, whose chairman, Bob Woollard, said: ‘This dilution and unravelling of marriage has demotivated many ordinary loyal Conservative Party members and has undermined their years of hard work for something they believed in. It makes winning the next election virtually impossible.’
Hundreds of Muslim leaders have also attacked the gay marriage plans in an open letter to Mr Cameron, Labour leader Ed Miliband and Liberal Democrat leader and Deputy Prime Minister, Nick Clegg.
More than 500 community leaders and imams signed the letter which stated ‘marriage is a sacred contract between a man and a woman that cannot be redefined’.
