The coalition's Queen's Speech relaunch appears under threat after Conservative MPs threatened to rebel en masse against its centrepiece, Lords reform.
Last night's meeting of the backbench 1922 committee saw normally loyal Tories speak out against the proposals to make parliament's second chamber at least 80% elected.
A Downing Street spokesperson insisted: "Lords reform was in our manifesto and the manifesto of the other two parties. It was a crucial part of the coalition deal. We are not planning to let this blow us off course."
But MPs warned after yesterday's meeting that the potential rebellion would be "off the scale" and make "Maastricht look like a tea party". Up to seven junior ministerial aides are thought to be prepared to resign over the issue.
Only one MP, Gavin Barwell, was prepared to speak in favour of the reforms.
Peers are expected to do everything they can to oppose the changes, forcing the coalition to use the Parliament Act. That means cross-party consensus will be critical to securing a majority in the Commons.
Mr Clegg and his Conservative constitutional reform minister Mark Harper have resisted calls for a referendum on the changes.
They argued in evidence to the Commons' political and constitutional reform committee yesterday that the 1999 barring of hereditary peers from the Lords, a similar change to the composition of the upper House, did not require the public's backing.
Labour is also pushing for a referendum, however. The joint committee of MPs and peers which has been scrutinising the coalition's draft legislation will deliver its verdict on Monday.
Ed Miliband today said he believed the public needed to be asked about the change as it represented a "major constitutional change".
"I think the best way of making sure that House of Lords reform happens is by giving people a say," he said.
"We'll obviously see what the committee that has been set up to look at this says on Monday but Labour's view – which we expressed in our manifesto – is we want a democratically elected second chamber, we wanted a referendum to make that decision and I hope the government will listen to what people are saying and hopefully what parliament will be saying."
Internal resistance from within the Conservative party is now emerging as another major threat to Lords reform, however. The changes may be critical to the Liberal Democrats' ongoing commitment to the coalition but many Tory backbenchers are now considering calling Nick Clegg's bluff.
One MP told the Mail newspaper that the PM "should tell Nick Clegg that if he wants to fight an election now he will have seven MPs left and they can all come to work in Simon Hughes' taxi".