David Cameron was facing a growing backlash from Tory MPs tonight over his plans to make it more difficult for the opposition to force a general election if the new coalition Government is defeated on a vote of confidence.
Senior Conservative backbenchers branded the measure - designed to seal the coalition pact with the Liberal Democrats - as "constitutionally incoherent" and a "recipe for anarchy".
Mr Cameron insisted today that the proposals - which would see him surrender the traditional right of the Prime Minister to choose the timing of the election - actually represented a "big giving up of power".
However Tory MPs opposed to the plan insisted they were "extremely hopeful" that ministers would be forced to re-think the whole idea.
Meanwhile, Mr Cameron also found himself under fire from his Lib Dem allies, with newly appointed Equalities Minister Lynne Featherstone sharply criticising the lack of women in the Government, saying: "We must do better."
The plan for five-year fixed-term Parliaments was a key plank of the coalition deal hammered out by the Conservative and Lib Dem negotiating teams following the General Election stalemate.
In order to reassure both sides that neither party could cut and run in order to force a fresh election when it suited them, it was agreed there should be a binding resolution of the House requiring the support of 55% of the MPs if Parliament is to be dissolved early.
However, Conservative MPs have now joined with Labour in strongly condemning the proposal, warning that it undermines the principle that a government can be forced out by a simple majority vote in the Commons.
Veteran Tory Richard Ottaway, who is one of the frontrunners for the chairmanship of the Conservatives' 1922 Committee of backbenchers, said it would "undermine the primacy of Parliament".
"It's constitutionally incoherent. Unless it can be clarified, it's not acceptable," he said.
Another senior Tory, Christopher Chope, said the plan had been "cobbled together" without properly consulting MPs. It should have been sufficient, he said, for Mr Cameron to give Lib Dem leader Nick Clegg an assurance that he would not call a snap election as "a matter of honour and trust".
"If the present Government was to lose its majority in Parliament and wasn't able to operate as a minority government because it didn't enjoy the confidence of a sufficient number of MPs, then what is being suggested is that it would be able to carry on. That would be, basically, a recipe for anarchy," he said.
Charles Walker, the Conservative MP for Broxbourne, said the measure was being introduced simply for the "convenience" of the coalition.
"It is not the duty of Parliament to prop up this coalition. That is the duty of the coalition partners and if they can't make it work and if they lose the confidence of Parliament then we must have a general election. It is a simple as that. This is about the primacy of Parliament," he said.
Mr Cameron, in Scotland for his first meeting with First Minister Alex Salmond, insisted the measure was necessary to secure stability although he accepted that it would have to be debated in the Commons.
He said: "I'm the first Prime Minister in British history to give up the right unilaterally to ask the Queen for a dissolution of Parliament. This is a huge change in our system, it is a big giving up of power.
"Clearly, if you want a fixed-term Parliament you have to have a mechanism to deliver it. Obviously that is a mechanism that can be debated in the House of Commons, it can be discussed, but I believe that it is a good arrangement to give us strong and stable government."
However the danger for Mr Cameron is that the issue could provide a rallying point for Tory MPs who are unhappy with the pact with the Lib Dems and who would have preferred the Conservatives to govern alone as a minority administration.
A strong Tory vote against the plan in the Commons, could start to undermine confidence in the coalition.
Mr Cameron, meanwhile, was stressing his determination to see a "fresh start" in the relationship between the Government in Westminster and the Scottish administration.
He met the leaders of the three main opposition parties in Scotland and had talks with SNP leader Mr Salmond.
"However much we may disagree about issues, we should try to work together for the benefit of the whole of the United Kingdom and for the benefit of Scotland as well. That is what I'm determined to do," Mr Cameron said.
Around 200 demonstrators gathered to meet him as he arrived at the Scottish Parliament in Edinburgh. They chanted anti-Tory slogans and carried placards calling for an end to job cuts.
A constitutional expert said that plans for a five-year fixed term parliament would not allow a Prime Minister to stay in power if defeated in a motion of no confidence.
Durham University's Professor Gavin Phillipson said: "These plans have provoked much alarm and criticism for being 'dangerous', 'undemocratic' and 'a stitch up'.
"Some say that it would prevent a simple majority of MPs bringing down an unpopular government.
"But some critics are making a simple mistake. The proposed change will not affect the rule that a PM must resign if defeated on a motion of no confidence by a simple majority.
"This won't stop MPs being able to bring down an unpopular government, just as they can now."
Prof Phillipson, who teaches Constitutional Law at Durham Law School, said the plans related to a separate issue, which is when Parliament can be dissolved early.
He said: "At present, when a government is brought down, a dissolution tends to follow, leading to a general election.
"Under the proposed change, parliament would remain sitting and the political parties would have to see if they were able to form a new government - either a minority administration supported by a 'confidence and supply' agreement or a new coalition with a new Prime Minister.
"The idea behind the 55% rule is to make sure that one party on its own can't trigger a fresh general election."
Prof Phillipson said that the legislation for the Scottish Parliament provided a model for change.
Terms for the Scottish Parliament are fixed, but there can be an early dissolution if members vote by a two-thirds majority to dissolve it or where the First Minister has died or resigned, which he must do if defeated on a motion of confidence.
In such a case, there is no automatic dissolution and the parties in parliament have 28 days to seek to form a new administration.
If they cannot, then the Scottish Parliament is dissolved.
Prof Phillipson added: "There's nothing inherently undemocratic in having a higher than 50% threshold for ending a parliament.
"It's simply designed to produce greater stability and stop a Prime Minister, using his whipped majority, calling a general election to suit his own convenience."