Pressure was mounting on Prime Minister David Cameron today to give Tory MPs a free vote in Monday's Commons debate on an EU referendum, when he faces the likelihood of the biggest rebellion of his 18 months in power.
At least 61 Conservative MPs have now signed a motion calling for a referendum on whether the UK should remain in the EU, leave or renegotiate its membership, and one prominent backbencher predicted the total number of rebels could top 85.
Mark Pritchard, secretary of the Tory backbench 1922 Committee, called on the PM to implement a non-binding single-line whip in Monday's vote, which would allow MPs to back the motion without facing disciplinary action.
Downing Street made clear this morning that Mr Cameron expects all Conservative MPs to oppose not only the referendum motion - triggered by a petition of more than 100,000 public signatures - but also two amendments tabled in the hope of finding compromise.
Camborne and Redruth MP George Eustice, who as Mr Cameron's former press secretary is seen as being close to Downing Street, is proposing that a referendum should be held only after the Government has completed the process of renegotiating the terms of UK membership with the EU.
And Watford MP Richard Harrington has tabled an amendment which backs the Government's commitment to hold a public vote on any future transfers of power to Brussels, and calls on ministers to negotiate the return of some powers now.
Speculation that Downing Street would give Tory eurosceptics the green light to back the Eustice amendment in the hope of minimising the size of any rebellion on Monday was scotched by Mr Cameron's official spokesman today.
"We do not think either of these amendments are in line with Government policy," said the spokesman. "We have a policy set down in the coalition agreement and we expect people to support that policy."
Mr Pritchard warned the PM that he could be defied by more than a quarter of his 306 MPs if he attempts to impose a three-line whip, and urged him to take a more relaxed approach.
"From discussions I have held over the last 24 hours, if the amendments to the EU motion are not selected by Mr Speaker, the number of Conservative backbench MPs backing the main referendum motion could rise to above 85 MPs," said Mr Pritchard. "The Government should think again and allow a one-line whip."
Among those planning to defy the whip is Stewart Jackson, the parliamentary aide to Northern Ireland Secretary Owen Paterson.
Mr Paterson today said he had not heard from his parliamentary private secretary since Mr Jackson announced his willingness to forfeit his unpaid Government job by joining the rebels.
One of the leading advocates of a referendum, Bernard Jenkin, said that the proposed amendments should be rejected because they would mean Parliament was not debating the issue to which petitioners put their signatures.
"There is a great danger that Parliament will emerge from this looking very out of touch if the House is not to debate the original motion, or at least something which reflects its spirit," Mr Jenkin warned in an open letter to Mr Eustice.
Independent MEP Nikki Sinclaire, who gathered the original petition, branded attempts to whip MPs on the issue undemocratic.
Announcing that she had gathered a further 20,000 signatures to hand to Mr Cameron on Monday, Ms Sinclaire said: "We've forced them to have the debate, but they have imposed whips. This shows complete contempt for the people who gave them their positions.
"They may think this will just fizzle away, but I will organise another petition and another and another until the British people get the right to choose who governs them... This is no longer about membership of the undemocratic EU. It is about whether democracy still functions in the UK."
Meanwhile, there were signs of dissent on the Labour side over leader Ed Miliband's order to vote against the referendum.
Former Labour whip Graham Stringer - who has signed the motion and intends to vote in favour of a referendum - told the BBC: "I think it is a mistake of all three party leaders when the public are clearly aching for a say on Europe to say, no, you can't have it."
And another signatory, former minister Kate Hoey, said: "This is an opportunity to stand up for the 75% of voters - including millions of Labour voters - who are demanding a referendum.
"People will be disappointed that Labour is taking the same line as the establishment Government parties in not allowing a vote, but this issue will keep returning until the public's voice is heard."