The coalition government’s move to make it harder to dissolve Parliament is a "constitutional outrage", ex-Transport Secretary Lord Adonis has said.
The Lib Dem-Tory plan will mean that 55% of MPs must approve such a move to get it through the House of Commons. A simple majority is currently enough.
Labour’s Lord Adonis said it raised doubts over the coalition’s legitimacy.
But Lib Dem Andrew Stunell, who helped frame the deal, said it was needed to prevent an "ambush" on the Tories.
The coalition agreement between the Lib Dems and Conservatives promises a "strong and stable" government, with elections held on fixed dates every five years.
‘Ganging up’
The raising of the threshold for a dissolution vote is intended to prevent a move to hold an election earlier than that.
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.
Lord Adonis said: "This is a brazen attempt to gerrymander the constitution which calls into question the legitimacy of the coalition from day one.
"If the legislation ever gets to the House of Lords, it will meet opposition of an intensity and bitterness not seen for many years. This is a constitutional outrage."
However, Mr Stunell, the Lib Dem MP for Hazel Grove, told BBC Radio 4’s PM programme: "What the prime minister has given up with a fixed-term parliament is the right to go to the Queen at any moment and just call a general election. Obviously that’s what a fixed-term parliament stops.
"On the other hand, if your threshold for a special case is only 50%, in theory it would be possible for the Tories to be ambushed by other parties, including the Liberal Democrats, ganging up against them…
"Although nobody in the partnership has any intention of doing any such thing, it was a small matter for us to say ‘No, we accept your concerns and if we raise that threshold to 55%.’
"That gives you the safeguard you want and that’s the way we’ve proceeded."
Charles Walker, Conservative MP for Broxbourne, said: "It is for Parliament to decide when it’s lost confidence in the government and I think we have to look at this very closely…
"This is perhaps just a little too much for our unwritten constitution to bear."
He added: "Parliament actually runs this country, not the prime minister. Over the past 100 years, Parliament has given away huge powers to the prime minister.
"We have a quasi-presidential system here, without the checks and balances. This would be the loss of an enormous check."
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