Fixed five-year parliamentary term will tie both leaders' hands

Neither the Conservatives nor the Liberal Democrats will be able to force the collapse of the coalition, leading to an early general election, under the terms of the legislation introducing a five-year fixed-term parliament.

The detailed text of the coalition agreement says a decision to trigger a general election can only be made if 55% or more of all MPs in the House of Commons vote in its favour. The threshold was seen yesterday as a safeguard for the Lib Dems, as it means the Conservatives cannot by themselves vote to end the coalition and go to the electorate at a time chosen by David Cameron alone.

However, the safety of the 55% limit was questioned last night. Scott Styles, a senior lecturer in law at Aberdeen University, described the move as "dangerous" and said: "This is a major and fundamental alteration in our constitution; what is being changed is not a right of the PM but a power of the Commons. The British constitution is very simple: he who commands the confidence of the House is PM, he who loses that confidence must resign.

"I simply do not see how such a rule is credible or can be enforced: a majority is a majority is 51%; not 55% or 60% or 80%."

The agreement outlines the steps to introduce a five-year fixed-term parliament and specifies the next general election will be on the first Thursday of May 2015. A "binding motion" is to be introduced into the Commons "in the first days", followed by the necessary legislation. The effect is to remove the prime minister's power to determine the date of the election within the conventional five-year term.

Cameron confirmed the move would happen. "This is not an aspiration, this is going to be very early legislation," he said. "We want that fix, because we think that that would make for good and strong and determined government that can act in the long term. Fixed parliaments is something that has been knocking around in terms of political reform for a long time, and I think that now is the moment to do that."

Clegg will deputise for Cameron at prime minister's questions when he is out of the country. The Lib Dem MPs will sit on the government benches. Many other details of Westminster life have yet to be worked out. Policy disputes between Conservative and Lib Dem ministers will be referred up to Cameron and Clegg to resolve between them. As deputy prime minister, Clegg will chair many cabinet committees, and see all the papers.

But both party leaders stressed that outside Westminster the parties would operate as independently as ever, especially at byelections such as the Thirsk and Malton contest due on 28 May. "We are not merging our two parties, so we'd expect our parties to put up candidates and to campaign in that intensely reasonable way that we always do," said Cameron.

However, he did expect the two candidates in Thirsk and Malton would show a commitment to the coalition.

Whitehall sources confirmed that Clegg, whose office as deputy prime minister will be based in the Cabinet Office, will take over responsibility for political and constitutional reform from the justice secretary. As well as changes to the electoral system he will oversee reform to how parliament operates and holds the government to account. The changes include MPs electing select committees by secret ballot, backbenchers gaining control over the timetabling of government business within three years, and the public being given the right to have a particular issue debated in the Commons.