Tory MPs in the Westcountry have ramped up the pressure on the Government over Lords reform by signing a rebel letter opposing the controversial shake-up.
Usually loyal Gary Streeter for South West Devon is among around 70 Conservatives at Westminster to put his name to warning the changes threaten to "pile a constitutional crisis on top of an economic crisis".
The rebels, who also include Neil Parish (Tiverton and Honiton), Geoffrey Cox (Torridge and West Devon), George Eustice (Camborne, Redruth and Hayle) and Ian Liddell-Grainger (Bridgwater) are calling for the House of Lords Reform Bill to be given the "full and unrestricted" scrutiny it deserves rather than the 10 days being offered by the Government.
The Bill proposes removing the final hereditary peers from the Upper House and creating a 450-member, 80% elected second chamber.
If all of the signatories oppose tonight's crunch vote on the timetable for the Bill, which restricts detailed debate to 10 days, it is set to lead to the Government's first significant defeat.
Losing the so-called "programme motion" would pave the way for a drawn-out parliamentary battle and leave the legislation vulnerable to being "talked out" by opponents.
This would heighten tensions within the Coalition being the remaining centrepiece of Liberal Democrat constitutional reform plans, following defeat in last year's referendum on voting reform.
The letter states: "We come from all sides of the Conservative Party and are writing as reformers to express our serious concern at the current proposals to create an elected House of Lords.
"The Lords Bill is a measure of profound constitutional significance. Over time it will change the institutions by which we as a nation are governed.
"It threatens to pile a constitutional crisis on top of an economic crisis."
It goes on to warn that the changes would undermine the primacy of the Commons and lead to "legislative gridlock".
The new chamber would be "less expert and significantly more expensive than the present one", it says.
Mr Streeter said: "To impose a directly elected House of Lords on us is a huge mistake. It would exclude wise and experienced people who have a lot to offer but will never stand for election in a million years.
"As soon as we have an elected second chamber they would want power and very quickly we would have constitutional tensions that would lead to gridlock.
"The House of Lords is a revising chamber only."