Gordon Brown is struggling to head off a Commons defeat tonight over his plans to allow police to detain suspected terrorists for up to 42 days without charge.

The Prime Minister took personal charge of the last-minute arm-twisting in a desperate attempt to avert a defeat that would trigger a crisis of confidence in his leadership. With more than 40 Labour backbenchers threatening to vote against the Government, one close Brown ally claimed it was “looking grim”.

Mr Brown’s fate could lay in the hands of about 10 wavering Labour MPs, who oppose 42-day detention but may pull back from voting against it because defeat would inflict further damage on the party’s standing with voters.

Ministers were forced to plead with members of the hard-left Campaign Group, some of whom Mr Brown has met three times, and the Democratic Unionist Party, who normally vote with the Conservatives and whose nine MPs could hold the balance.

The Prime Minister even risked a rift with George Bush on the eve of the US President’s visit to London by promising Labour left-wingers he would back the lifting of EU sanctions against Cuba if they supported him on 42 days. Mr Bush wants the sanctions, which include a ban on high-level visits by Cuban officials, to be maintained but some EU countries, led by Spain, want them abolished to encourage the new leadership in Cuba.

“It was a tempting offer but I have decided that human rights in Britain are more important,” said one left-winger who was lobbied by Mr Brown.

Whips are pulling out all the stops. One Labour MP, John MacDougall, who is ill and rarely attends the Commons, will travel from his Glenrothes constituency to support the Bill. Whips are warning potential rebels that defeat would inflict terrible damage on the Government. But Downing Street denied that Mr Brown would call a vote of confidence if he suffers his first Commons defeat.

The Government suffered a triple setback on the eve of the crunch vote. It was accused of “sexing up” the need for police to question terror suspects beyond the current 28-day limit.

Ministers have argued that during Operation Overt, which exposed an alleged plot at Heathrow airport in 2006, evidence came to light only at the end of the 28-day period. They have claimed it proves the police are “up against the buffers” and that a longer period will be needed in future terrorism cases.

But the human rights group Liberty accused the Government of making “inaccurate and misleading” statements about the operation. A lawyer on the case has told Liberty that the evidence used to charge two suspects at the end of the 28-day period was obtained by police within four and 12 days respectively. The lawyer claimed that, during the last 15 days of detention, the interviewing of both suspects tailed off dramatically, lasting an average of 10 and 16 minutes a day respectively.

Shami Chakrabarti, the director of Liberty, said: “I am shocked, angry and more than a little disappointed to find ministers have repeatedly sexed up the operational pressures on the existing 28-day detention limit. I hope the similarities with the infamous Iraq vote will not be lost on Labour MPs.”

David Davis, the shadow Home Secretary, said the revelations were the “final nail in the coffin of the Government’s case for 42 days”. He said: “It is now clear that, in the most complex counter-terrorism case in British history, we were nowhere near the 28-day limit. It demonstrates all too clearly why the prosecuting authorities say that they can manage comfortably with the current 28-day limit.”

The Muslim Council of Britain launched a strongly worded attack on the proposed extension, warning it would damage community relations and undermine Britain’s moral authority around the world. Muhammad Abdul Bari, its secretary general, said: “This legislation will be counterproductive and will play into the hands of extremist groups.”

Andrew Dismore, chairman of Parliament’s joint human rights committee, said the Government’s definition of a “grave, exceptional terrorist threat” was “extraordinarily broad and includes events or situations which fall well short of constituting a genuine emergency in any meaningful sense of that word”.

Jacqui Smith, the Home Secretary, will try to win over wavering Labour MPs during today’s Commons debate by promising compensation for terror suspects held beyond the28-day limit and then released.

Ten Labour MPs told The Independent they were still making up their minds how to vote. Graham Stringer, MP for Manchester Blackley, said: “So far, the Government has failed to persuade me there is evidence they need the 42 days. I will listen to what they have to say”

How the detention period would work

*Day 1

Police arrest men suspected of plotting a mass attack.

*Day 3

Senior officers apply to magistrates to hold suspects for up to seven days under the 2000 Terrorism Act.

*Day 7

Crown Prosecution Service applies for a further seven days’ detention.

*Day 14

Application made to High Court for another seven days’ detention.

*Day 21

Application for further seven days.

*Day 27

Home Secretary says there is “grave exceptional terrorist threat” to the country. Director of Public Prosecutions (DPP) and police agree longer detention is justified.

*Day 28

Home Secretary signs order bringing 42-day limit into force under 2008 Counter-Terrorism Act. Commons Home Affairs Select Committee, Parliamentary Joint Human Rights Committee and Parliamentary Intelligence and Security Committee are informed.

*Day 29

Home Secretary tells Parliament that emergency powers activated.

*Day 35

Deadline for extended detention period to be approved by the Commons and the Lords. Detention for further seven days needs new consent from DPP and judge.

*Day 42

Suspects to be charged or released.

*Day 58

Emergency higher detention limit expires.

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