A senior Liberal Democrat peer today warned Conservatives that they must back coalition plans to reform the House of Lords - despite a threatened revolt by Tory backbenchers.
Conservative MPs were reported to have lined up to condemn the proposals - set to form the centrepiece of next month's Queen's Speech - at a "stormy" meeting last night of the party's backbench 1922 Committee.
A number of ministerial aides were reported to have indicated they would rather resign than support legislation for a largely- or wholly-elected upper chamber, which they fear could threaten the primacy of the House of Commons.
However, senior Lib Dem peer Lord Oakeshott insisted today that the plan was part of the coalition agreement and should be supported by MPs of both parties.
"Our vast unelected House of Lords is overstuffed with complacent dinosaurs," he said.
"Electing both Houses of Parliament is a simple democratic principle, it's in all three major parties' general manifestos and the coalition agreement.
"So all coalition MPs should back it - and Labour mustn't drag their feet on this long overdue reform."
The issue of Lords reform threatens to stretch coalition unity to breaking point in the coming months.
Deputy Prime Minister Nick Clegg has insisted it is an essential part of the coalition's constitutional reform programme, while Lord Oakeshott has previously warned Lib Dem peers could wreck Tory plans to cut the number of MPs if it does not go through.
On the Conservative side, many backbenchers - already frustrated at the Lib Dems' influence within the coalition - are adamant they are not prepared to support a measure which could, they believe, upset the whole balance of the constitution.
Downing Street said David Cameron remained committed to reform of the House of Lords.
"Both the Prime Minister and the Deputy Prime Minister are committed to Lords reform," a No 10 spokeswoman said.
She added that all members of the Government would be expected to support the legislation when it comes to Parliament.
"If it is Government policy, one expects the Government to support it," she said.
She said ministers were "not persuaded" by calls for a referendum on the issue.
Conservative MP Peter Bone said many of his colleagues on the Tory backbenches were dispirited at the prospect of Lords reform being placed at the heart of the Government's agenda in next month's Queen's Speech.
Opposition to the change came from all sides of the party, though it was unclear how many would actually be prepared to rebel when it came to a vote in the Commons, said Mr Bone, who was not at last night's 1922 Committee meeting.
But he said the Government may face defeat on the programme motion setting out the timetable for the Bill's passage through the Commons, which could "gum up" the legislative agenda for the coming year.
He predicted Mr Cameron would offer compromises to backbenchers to smooth the process, with the unspoken assumption that a Tory majority at the next election would lead to the issue being kicked back into the long grass.
"It is a huge problem for the Prime Minister and he knows it's a huge problem," said the Wellingborough MP. "It certainly could bring the coalition down, though whether it will I don't know.
"The one thing the Liberal Democrats are demanding is Lords reform because Nick Clegg has failed on everything else, and if he gets this then he will go down in history as the Liberal leader who achieved something they've been trying to do for 100 years.
"Presumably there will be some sort of sweeteners, and the Prime Minister will say we will reach some sort of compromise."
Mr Bone said Tory backbenchers were "incredulous" that the Government was prepared to devote large amounts of time and effort over the coming year to an issue which was of concern to few voters.
"We are told we can't have a referendum on Europe because nobody is interested in it, which is nuts," he said. "But something that people are genuinely not interested in is going to demand parliamentary time."
Katie Ghose, chief executive of the Electoral Reform Society, said: "We can understand why unelected Lords might have little time for accountability or manifesto commitments. These MPs have no such excuses.
"Every one of these wannabe-rebels ran for Parliament on a promise of Lords reform.
"Now they seem prepared to sacrifice their own Government's entire legislative programme to bury it.
"It sends a strong signal to their constituents that their manifestos weren't worth the paper they were printed on.
"The Government is right to stick to its guns. No party went into the last election defending the status quo, and if Lords reform is ever to see the light of day it will require all parties to stay committed."
Labour leader Ed Miliband said he was firmly behind proposals for a fully elected House of Lords.
Speaking while on a local government election campaign in Merthyr Tydfil, south Wales, today, he said: "A referendum on the issue is a good idea, and we have said this previously in our manifesto.
"But I don't quite understand why the Government is setting its face against giving the people a say.
"We have made our position in wanting a referendum for a 100% elected House of Lords very clear.
"There's a report from an independent committee on Monday, let's see what that report has to say.
"Am I surprised at the way some Tories have threatened a rebellion against the issue? Look, that's a matter for them and I will leave the Tories' problems to themselves.
"There are lots of more important issues facing our country right now such as health, education and jobs."
Labour former home secretary David Blunkett said: "The implementation of Nick Clegg's proposals to change our centuries-old system of parliamentary governance is ill-thought through and will have wide-ranging repercussions.
"An elected senator, having been voted in rather than appointed, would not feel bound by the customs and practice that have applied to an unelected chamber - and will undoubtedly come into conflict with the House of Commons.
"Meanwhile, the so-called senators will be highly unaccountable - elected to serve non-renewable 15-year terms and unanswerable to the voters in the normal fashion of an elected chamber."
Mr Blunkett said: "This dog's dinner of a proposal should be thrown out and, in any case, any proposal for a referendum should take place before legislating on the detail and not afterwards."