Rebel Tory MPs opposed to coalition plans to introduce an elected House of Lords have put their names to a letter warning the Bill threatens a "constitutional crisis".
Under measures contained in the Lords Reform Bill the current unelected chamber will be replaced with an 80% elected 450-member House.
The letter sent to other Tory MPs on Monday morning by the rebels reads: "We come from all sides of the Conservative Party, an are writing as reformers to express our serious concern and the current proposals to create an elected House of Lords.
"What is now proposed will undermine the primacy of the Commons, with competing chambers which will lead to legislative gridlock.
"It will create hundreds of unaccountable new elected politicians at a time when we as a party are committed to reducing the cost of politics.
"And it will produce a chamber which is less expert, less diverse and significantly more expensive than the present one.
Introducing a Commons debate on Monday afternoon, Nick Clegg said the Bill was about fixing a "flawed institution".
"We believe that the people who make the laws should be chosen by the people subject to those laws," he said.
"That principle was established in Britain after centuries of struggle, and it’s a principle we still send our servicemen and women halfway across the world to defend."
Clegg was quite surprisingly given very little help from his Lib Dem colleagues on the coalition benches, who sat quite glumly in silence as the deputy prime minister faced repeated attempts from Tories to interrupt him.
Hostile Tories including Conor Burns openly mocked Clegg at some points, particularly when the he insisted that a referendum on Lords reform wasn't required.
Other Conservative backbenchers sat chuckling, clearly enjoying watching Clegg struggle to make headway in his speech.
While the reforms themselves are expected to clear their first parliamentary hurdle with Labour support, up to Tory rebels are said to be ready to join with the Opposition to throw out the timetable for further stages.
The rebel letter asks fellow Tories to join the rebels in giving the Bill the "unrestricted scrutiny it deserves".
Among the signatories to the letter are senior Tory backbenchers including the chairman of the defence committee James Arbuthnot, the chairman of the public administration committee Bernard Jenkin and former foreign secretary Malcolm Rifkind.
Losing the vote on the timetable leaves the Bill vulnerable to being 'talked out' by opponents who want to torpedo the plans.