David Cameron is today hurtling towards a humiliating Commons defeat over House of Lords reform, with up to 100 Conservative MPs set to defy him in a crucial vote.
Tory MPs are poised to help Labour defeat a motion to set a strict timetable for debate on the reforms, which would plunge coalition business in the Commons into chaos.
At least 70 Conservative backbenchers have signed a letter warning the plans for a mainly elected upper chamber threaten a "constitutional crisis" and calling for the Bill to be given "full and unrestricted" scrutiny.
The rebels include grandees and select committee chairmen - among them Bernard Jenkin, John Whittingdale, James Arbuthnot and Bill Cash - as well as the former shadow home secretary David Davis.
Loyalist Tory MPs are said to be in a mood of dismay and despair after Deputy Prime Minister Nick Clegg gave what they claimed was a lacklustre performance as he opened a two-day Commons debate on his Lords shake-up.
Defeat for Mr Clegg tonight would throw the coalition government into one of its gravest crises.
The Deputy Prime Minister has warned David Cameron in private that he may block his plans to reduce the House of Commons from 650 to 600 MPs if he believes that the Tories have failed to do enough to persuade their MPs to support his reforms.
In a speech in which he was constantly interrupted by Tory MPs hostile to the reforms , Mr Clegg said he was simply trying to reform a "flawed institution".
"We believe the people who make the laws should be chosen by the people subject to those laws," he said.
But in an ominous intervention, Graham Brady, chairman of the influential backbench Conservative 1922 Committee, said the Bill was being driven by "calculations of party advantage".
"We spend too much time here pursuing party advantage," he said. "To do so in changing our constitution would not be just wrong, it would be contemptible."
Mr Clegg also received a withering rebuke from Nicholas Soames - grandson of Sir Winston Churchill - who told him to stop quoting his grandfather in support of Lords reform.
"As he grew up through his political life, he dropped those views and had great reverence and respect for the institution of the House of Lords - something I suggest he should have as well," said Mr Soames.
Earlier, veteran Conservative Edward Leigh confirmed that many Tories wanted the Bill to fail in the hope it would bring down the coalition.
"Why don't we all vote against House of Lords reform tomorrow and then we will end the sad life of this coalition, we can have a General Election, a Conservative government," he said to Tory cheers.
