Leading right wing Tory backbench critic David Davis claimed that the government has “a huge battle” on its hands if it wants to press the reforms to the upper house through.
With a two-day debate set to begin on the issue starting today, it is expected that Tory rebels will join with Labour to vote against a motion which will mean the bill could be talked out without a vote.
Mr Davis warned that it would mean that the bill could in effect throw the whole coalition government legislative programme into chaos.
Despite the threat senior Lib Dems, who are desperate to push the reform through and make the Lords 80 per cent elected, insisted that they could still win.
However, despite Business Secretary Vince Cable saying that defeat would not break up the coalition, senior Lib Dems have warned that they will block key Tory reforms on education in England and other areas as well threatening to stop the boundary changes which would remove some of Labour’s in-built advantage in the electoral set-up.
Mr Cable said: “We are not talking about walking away. That isn’t an issue.
“But there is absolutely no reason why this vote should be lost. All three parties agree that we should have an elected House of Lords, that the present system is completely unsustainable.
“There is no point engaging in a hypothetical debate about something I don’t think will actually happen.”
Asked about the threat of “broader consequences” of defeat by the Deputy Prime Minister Nick Clegg’s outgoing strategy director Richard Reeves, Mr Cable said: “We are not threatening anybody. We will vote on those individual merits when the time comes. We are working with a businesslike way with our Conservative partners. We disagree on some things but we get on with things.”
Mr Davis said most Tories saw House of Lords reform as “a sop to the Liberals to give them something to be elected to when they get wiped out at the next election”.
He said: “That’s the internal friendly dynamic of it.
“It is going to be a huge battle. It is a two-day debate and it is a battle within the coalition as much as anything else. The truth of the matter is the government can’t control it.
“There could be as many as 100 rebels. If the vote had been taken on Friday they would have lost hands down. If they choose to prioritise this is and try and appease the Liberals, that’s what happens.”
A series of former Tory cabinet ministers also added their weight to opposing the reforms which would see them ejected from the Lords.
Among them two former chancellors, Lord Howe and Lord Lamont, were among senior Tory peers adding to the pressure on Prime Minister David Cameron by signing the cross-party letter defending the upper chamber.
It was reported over the weekend that the cross-party group of 36, also including ex-Commons speaker Betty Boothroyd, said the existing House was “a vast reservoir of talent and experience, which complements the more youthful and vigorous House of Commons without ever being able to threaten it”.
The reforms would “remove the unambiguous democratic mandate the House of Commons currently enjoys”, they added.
Other ex-cabinet ministers who signed the letter are reported to be Lord Brooke, Lord Forsyth, Lord Jopling, Lord MacGregor, Lord Mayhew, Lord Lang and Lord Jenkin.
Lord Forsyth of Drumlean, a former Conservative cabinet minister under Margaret Thatcher, described the legislation as a “ridiculous bill”.