David Cameron has defended plans to change the rules on how an election is called, saying they will help the stability of his coalition government.
The Lib Dem-Tory deal agrees to fixed-term parliaments which can only be dissolved with support from 55% of MPs.
Labour MPs say it is a "fix" as 50% of MPs plus one can currently trigger a no confidence vote in the government.
Mr Cameron said he was the first prime minister to give up power to call an election and this was a "good change".
Speaking on a visit to the Scottish Parliament, Mr Cameron said there needed to be a "mechanism" to dissolve Parliament and the procedure he was proposing would help to secure a "strong and stable government" over the next five years.
"It is an important change and one I think should be welcome," he said, on a visit to Scotland to meet First Minister Alex Salmond and other party leaders.
"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."
Downing Street says Labour put through fixed-term laws in Scotland requiring 66% of MSPs to dissolve Parliament.
Five-year term
The prime minister has the power to ask the Queen to dissolve parliament at any time within a five-year period – which critics say benefits the ruling party.
The new coalition government has instead proposed to have five-year fixed term parliaments. But in a coalition agreement drawn up between the Conservatives and Liberal Democrats, it says legislation "will also provide for dissolution if 55% or more of the House votes in favour".
Currently a majority of MPs – 50% plus one – are needed to carry a vote of no confidence. In 1979 James Callaghan’s minority Labour government fell after losing a confidence vote.
Four senior Labour figures – and two Conservative backbenchers, Christopher Chope and Charles Walker, have expressed concern about the plans.
Mr Walker said: "This is perhaps just a little too much for our unwritten constitution to bear". And his colleague Mr Chope told BBC Radio 4’s World at One the coalition deal seemed to have been "cobbled together in quite a short space of time".
‘Totally unworkable’
He said: "It could mean in practice that 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 … that then what’s being suggested is that it would be able to carry on but that would be basically a recipe for anarchy, because it would mean that the government wouldn’t have a majority."
Former Transport Secretary Lord Adonis called it a "brazen attempt to gerrymander the constitution which calls into question the legitimacy of the coalition from day one".
Mr Straw said the plan was "completely undemocratic and totally unworkable" while Mr Blunkett described it as a "stitch-up".
Labour former Lord Chancellor Lord Falconer – a supporter of fixed-term parliaments told BBC Newsnight he feared it would result in a "zombie government" – as it would mean 53% of MPs could vote against a government but it would still continue until the fixed date.
But former Lib Dem MP David Howarth, a legal academic who drew up the original Lib Dem plans for a fixed-term parliament, told the BBC the vote of confidence and dissolution of Parliament were "entirely different things" and said Mr Straw was "totally confused".
In other countries with fixed-term parliaments, if a government lost a vote of confidence the parties would have to try to work out a new government within the fixed term, he said.
He said critics had got "entirely the wrong end of the stick" adding: "This dissolution vote, the 55% for a dissolution, is not the same as, for a vote of confidence."
‘Iffy politics’
A Downing Street spokeswoman said the old rule would still apply to no confidence votes – but should a government be defeated, it would not automatically trigger an election as a 55% vote would be required to dissolve parliament.
She said the details would all be debated and voted on in parliament and the former Labour government had put through the fixed-term legislation in Scotland which requires a 66% vote to dissolve parliament.
There is also some confusion among constitutional experts. Professor Peter Hennessy, of Queen Mary University of London University, told the BBC it looked like "very very iffy politics indeed" and there was a "certain brutal efficiency… about traditional confidence votes that one is enough and confidence votes under our system trump everything else".
But Professor Robert Hazell, director of the Constitution Unit think tank, told the BBC he understood the 55% threshold was intended to prevent the government from calling an early election without the consent of both coalition partners – effectively protecting the Lib Dems.
"It certainly won’t prevent the opposition from tabling confidence motions on which the normal threshold of 50% will and should, continue to apply."
Read a round-up of analysis by lawyers, MPs and academics
The Conservatives currently have 306 out of 649 MPs – a 47% share.
One seat, Thirsk and Malton, is empty, pending a by-election on 27 May, while Sinn Fein’s five MPs have not taken the oath of allegiance allowing them to sit in Parliament.
It would be impossible for opponents, even if fully united, to muster the 55% needed to dissolve Parliament, unless at least 16 Tories rebelled against their party leadership.
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