A group of Tory MPs will attempt to force a Commons vote over a referendum on the UK's European Union membership next week in a fresh blow to David Cameron's authority.
The MPs are seeking to engineer a vote on an EU referendum by tabling an amendment to the Queen's speech.
The Prime Minister has been struggling to rein in his party over Europe after Ukip put in a strong showing in local elections.
Backbencher Peter Bone, who tabled the measure in the Commons, said he hoped to show the Tory leadership that there was "significant parliamentary demand" to enshrine the referendum pledge in law before the general election.
Tory John Baron, who tabled the amendment along with Mr Bone, organised a letter signed by dozens of Conservative MPs demanding referendum legislation but was told that the nature of the coalition Government meant that was not possible.
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Mr Baron saidn that a vote on the amendment would be dependent on it being selected by Speaker John Bercow, but said he was hopeful it would be debated next Wednesday.
The amendment simply states that the House "respectfully regrets that an EU referendum bill was not included in the Queen's Speech".
Even if passed by a majority of MPs, it would not have the effect of forcing the Government to table a bill.
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Although Mr Cameron has presented a "credible" plan for a referendum in 2017, the level of public distrust of politicians' promises on Europe means it may not be "believable" to some of the electorate, said Mr Baron.
"Legislation in this Parliament is more believable than a manifesto promise," he said.
Mr Baron declined to say how many of the 100 Tory MPs who signed his letter would back the amendment next week.
As the names of those signing have not been released, it is possible that some of the MPs involved may not wish to take a public stance by rebelling in a vote.
Tory MP Nadine Dorries said she was signing the rebel amendment on her first day back in the party, after having the whip suspended for six months.
She declined to say how many other Conservatives would back the vote, but said it would be "more than it would have been a week ago" before the council elections.
The Mid-Bedfordshire MP welcomed yesterday's Queen's Speech, but said she and many of her colleagues were concerned about the level of voter unhappiness with Britain's relationship with the EU.
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The Prime Minister today lashed out at Tory "pessimists" demanding immediate withdrawal from the European Union.
Mr Cameron defended his approach of seeking fundamental reform and holding a referendum on membership after 2015 as "logical, sensible, practical.
He said: "I am faced as I do so, if you like, by two groups of pessimists.
"There are some pro-European pessimists who say, 'you have to, in Europe, simply sign up to every single thing that anyone in the EU suggests.
"You sign every treaty, you sign everything - there is no alternative'.
"I think they are completely wrong.
"The second group of pessimists say there is no prospect of reforming the EU, you simply have to leave. I think they are wrong too.
"I think it is possible to change and reform this organisation and change and reform Britain's relationship with it."
London mayor Boris Johnson has said that leaving the EU could be a "shot in the arm" for British democracy although he said he remained in favour of remaining part of the union.
He said: "My view has always been narrowly in favour of staying in, particularly for protecting British interests in the single market to avoid us being discriminated against.
"I now think that there is a real opportunity to get a better deal from Europe and that is what we need to do.
"And that is why the idea of a renegotiation with Europe is a very good one."
Tory MPs plotting EU Commons vote
- Hardline Eurosceptic MP Peter Bone has said he hoped to show the Tory leadership that there was "significant parliamentary demand" for the EU referendum pledge.
- Tory John Baron, who also tabled the amendment, organised a letter signed by dozens of Conservative MPs demanding referendum legislation.
- Former Cabinet minister Michael Portillo has followed ex-chancellor Lord Lawson in calling for the UK to leave the EU.
- Lord Lamont, another former Tory chancellor, joined the stampede of former Tory cabinet ministers saying it was possible for the UK to survive outside the EU.
- Nadine Dorries, the MP who was handed back the Conservative whip only on Wednesday, said she would be joining the rebellion.