Gordon Brown faces a humiliating parliamentary defeat over plans to allow police to hold terror suspects for up to 42 days without charge.
A survey of Labour MPs by The Independent has uncovered a growing insurrection. Only 34 votes are needed to defeat the detention plans and at least 38 MPs enough to wipe out Mr Brown's Commons majority of 67 are vowing to oppose controversial moves to extend the existing 28-day maximum detention period.
The scale of the rebellion will alarm Labour whips determined to hit the ground running next year after the Prime Minister's disastrous end to 2007.
It emerged as Sir Ken Macdonald, the Director of Public Prosecutions, delivered a damning verdict on Mr Brown's 42-day plans. He argued that the 28-day limit was working well, accusing ministers of wanting to pass laws based on a theoretical threat. "I think the basic point is whether you want to legislate on the basis of hypotheticals or whether you want to legislate on the basis of the evidence that we have acquired through practice," Sir Ken told BBC Radio 4's The World at One. "It seems to me that if you are legislating in an area which is going to curtail civil liberties to a significant extent, it is better to proceed by way of the evidence and the evidence of experience."
The struggle over 42-day detention, which ministers say is necessary because of the increasing complexity of terrorist conspiracies, is due to come to a head within two months. Conservatives, Liberal Democrats and minor parties have already vowed to oppose the moves, which means that Mr Brown risks losing his first Commons vote since taking the reins.
Earlier government suggestions of a 56-day limit have been dropped and Jacqui Smith, the Home Secretary, has launched a campaign to win support for the new proposal. She has stressed that there would be tough judicial and parliamentary safeguards on each occasion that the existing 28-day limit was exceeded.
But, although MPs praise her efforts to consult them, there is no sign of the rebellion abating.
MPs on the Labour-controlled Commons Home Affairs Select Committee spoke for many last week when they complained that ministers had failed to provide evidence that an extension of pre-charge detention was needed.
Potential rebels include a string of former ministers and senior backbenchers that goes beyond the "usual suspects" of Labour refuseniks.
Gwyneth Dunwoody, chairman of the Commons Transport Committee, said she could not support the proposals. She said: "Even the most stupid of us would like to see a bit of evidence."
Rebel MPs argue that the Home Office's attempts to consult over the measure have rebounded by producing a wave of criticism of the move from senior figures including Sir Ken, Lord Falconer of Thoroton, the former lord chancellor, and Lord Goldsmith, the former attorney general.
Just 8 per cent of respondents to a Home Office consultation exercise backed an extension.
The Liverpool MP Peter Kilfoyle, a former armed forces minister, said: "There's no evidence that more than 28 days is needed by police and the security services. All the key people are all quite satisfied with 28 days and that's where we should stick."
Chris Mullin, the MP for Sunderland South and a former foreign office minister, said: "In my time the number of days has gone from three to seven and 14 to 28 and I think that's quite enough."
Glenda Jackson, MP for Hampstead and Highgate and a former transport minister, said: "The Government's position seems to me absolutely illogical. It's not more time that is needed, it is more efficiency."
Fabian Hamilton, the Leeds North East MP, said he voted in 2005 for a 90-day maximum out of loyalty to the Government but would oppose 42 days. He warned: "We are eroding the liberties we hold so dear and that is what the terrorists want and we must resist that at all costs."
Alan Simpson, the Labour MP for Nottingham South, branded the proposal "crap". He said: "This could be part of a catalogue of self inflicted wounds by the Government."
If the proposals scrape through the Commons, a coalition of Liberal Democrats and Conservatives threatens to shred them in the House of Lords.
Ms Smith has said she will "continue to seek parliamentary consensus on the Government's proposal", adding: "As Home Secretary I am not willing to leave this potential risk to the security of the British people unaddressed."
The rebellion echoes Tony Blair's failed attempt in November 2005 to increase the pre-charge detention period from 14 to 90 days. It resulted in his first Commons defeat when 49 Labour MPs defied the Government, forcing ministersto compromise on the current 28-day limit.
The Independent's survey suggests the new revolt is on the same scale, presenting Labour whips with the daunting task of trying to talk round the critics. Alternatively the Home Office may have to produce further concessions, although attempts to highlight planned parliamentary and judicial safeguards have not so far won over many Labour MPs.
Shami Chakrabarti, director of the civil rights pressure group Liberty, said: "The Government must recognise that more and more MPs believe that extended pre-charge detention periods are divisive, counter-productive and simply will not wash in a democratic society."
Labour critics also warned that Mr Brown faces a tough new year battle with his own side over the Bill to ratify the European reform treaty, while legislation to promote social housing could reopening the bitter internal battle in the party over the future of council housing.
PM suffers more rebellions than any other leader
Gordon Brown was hit by a record number of revolts from his own backbenchers even in the first days of his premiership, new research has revealed.
Academics at Nottingham University found that more Labour MPs defied the government whips in the Prime Minister's first month than any other post-war premier.
Labour backbenchers defied the whips 43 times, with the first rebellion coming less than an hour after Mr Brown took over the reins of power. By contrast Harold Wilson enjoyed 16 months of unity before a lone Labour member rebelled over pensions policy.
Even John Major, whose premiership was marred by repeated backbench rebellions, lasted six days before his MPs defied the whips.
Professor Philip Cowley, an expert on parliamentary voting, said Mr Brown had suffered the largest single rebellion in the first month of any post-war premier.
The previous record was seven votes against John Major over Europe. Mr Brown suffered rebellions of 17 and 16 over pensions.
He said: "In itself these figures are not that alarming for the new Government. The number of rebels has not spread far beyond the list of the usual suspects. But they are proof that the rebellious behaviour seen under the Blair governments, which itself set a whole batch of records that the whips would have rather seen left alone, has not gone away."
He added: "The evidence is clear enough; Brown needs to treat his backbenchers with care."