Ministers are braced for a potentially damaging Commons defeat over Nick Clegg's plans for House of Lords reform.
The Deputy Prime Minister was given a rough ride by Tory backbenchers as he opened a two-day debate which could have profound consequences for the coalition.
Afterwards aides admitted their chances of winning the crucial vote on a timetable motion - limiting the time for debate in the Commons - were no better than 50-50. "It is close - unquestionably," said one aide to Mr Clegg.
Rebel Tories are threatening to combine with Labour to vote against the timetable motion - seen as essential if the House of Lords Reform Bill is not to be "talked out" by opponents making marathon speeches.
Although the Bill has the backing of the both the Conservative and Liberal Democrat leaderships, it is regarded as vital to the Lib Dems after their defeat in the referendum on AV voting reform for parliamentary elections.
Downing Street confirmed all members of the Government - including unpaid ministerial aides - would be expected to vote for the legislation.
However one opponent - Conor Burns, the parliamentary private secretary to Northern Ireland Secretary Owen Patterson - said he had written to the Tory Chief Whip Patrick McLoughlin putting his job "at his and the PM's disposal".
Earlier it emerged that around 70 Conservatives backbenchers had signed a letter warning the plans for a mainly elected upper chamber threatened a "constitutional crisis" and calling for the Bill to be given "full and unrestricted" scrutiny.
Signatories included a number of select committee chairmen - among them Bernard Jenkin, John Whittingdale, James Arbuthnot and Bill Cash - as well as the former shadow home secretary David Davis.
Opponents of reform argue that an elected House of Lords could undermine the traditional primacy of the Commons, leading to constitutional deadlock.