David Cameron is facing the first political resignations from his Government as an increasing number of Tory MPs are threatening to defy the whips and vote to hold a referendum on Britain's membership of the EU.
Monday's vote will be the first organised rebellion against Mr Cameron's leadership since he won control of the Conservative Party six years ago – and on the issue that tore apart the last Conservative government.
A total of 76 MPs – 60 Tories, and eight each from the Labour and Democratic Ulster Unionist parties – have signed the motion calling for a referendum that would give voters three choices – to stay in the EU on present terms, leave, or renegotiate the terms of the UK's membership.
Their chances faded when Ed Miliband announced yesterday that Labour MPs are being told to support the Government. And even if the rebels somehow won the vote without Labour's official support, the outcome is not binding.
The vote was originally scheduled for Thursday, when David Cameron and William Hague will be away at a Commonwealth summit in Australia, but the Government insisted on bringing it forward so that the Foreign Secretary can speak in the debate.
There were hints the Government was going to impose a three-line whip to deter half-hearted rebels, although Graham Brady, chairman of the influential 1922 Committee, urged it yesterday to allow a free vote.
Three Tories – the former leadership contender David Davis, and two new MPs, Priti Patel and Zac Goldsmith – will be joined by the Labour MPs Keith Vaz and Graham Stringer in a rally tomorrow which its organisers, from the People's Pledge Campaign, claim will be the largest ever held for a referendum on Europe.
Stewart Jackson, parliamentary aide to the Northern Ireland Secretary Owen Paterson, said that if the Government imposes a three-line whip on the vote, he will resign to vote with the rebels. Other parliamentary aides are expected to follow his example.
He told the BBC's World at One programme: "I will vote in favour of the motion and, in so doing, I will very likely relinquish my position... but some things are more important."
Mark Pritchard, secretary of the 1922 Committee, confirmed that he is also prepared to defy a three-line whip: "Ultimately, this is about country first, party second and career last."
Explaining why the Labour Party is backing the Government, Mr Miliband said: "It's not the right thing for Britain. It is not the right thing for jobs. It is not the right thing for growth... a referendum would create economic uncertainty our country does not need right now."