As Mr Blair prepared for a knife-edge Commons vote on the proposal tonight, he was accused of vetoing a plan by Charles Clarke, the Home Secretary, to compromise over the 90-day limit. On Monday, Mr Clarke announced he would propose an amendment to the Terrorism Bill reducing the limit below 90 days but after Mr Blair met the parliamentary party he decided to press on with his original plan.
It had been anticipated that the Government would attempt to secure a 60-day limit as a compromise. That may yet still be introduced tonight if the 90-day plan is defeated.
If Mr Blair's preference is voted down, it would be his first defeat in the Commons on a whipped vote since he came to power in 1997. It would signal more difficulties for his troubled leadership. There are growing strains between the Home Office and Downing Street ahead of the vote. One Home Office official said: "There is a feeling we might just get away with 90 days. There is going to be really strong pressure [on Labour MPs] on this one. There is bound to be last-minute lobbying."
Labour critics accused Mr Blair of playing party politics over the vote as some Tory MPs prepared to break ranks by backing a 90-day limit instead of the Opposition's policy to raise the present 14-day limit to 28 days. They may include the senior Tories Bernard Jenkin, Bill Cash and Ann Widdecombe, the former Home Office minister, who said: "It is wrong to hold a Dutch auction, frankly, over the number of days. If there are grounds for 90, let's go for 90. If there are not, let us not have it."
Gordon Brown, the Chancellor, piled more pressure on the Tories as he rallied behind the Prime Minister. He accused David Davis and David Cameron, the two Tory leadership contenders, of "short-term political opportunism".
Several Labour MPs said they had been persuaded to back 90 days. But government whips said the result remained "too close to call" and Mr Blair was warned that his tactics had provoked "immense anger" on the Labour backbenches. Labour rebels will join the Liberal Democrats and the Tory Opposition in voting for a 28-day detention limit.
Frank Dobson, the former health secretary, said: "Last week, the Home Secretary and Labour whips gave the impression that there would be a substantial reduction from 90 days. I think they did it in good faith. They have now gone back on it on instructions from No 10."
Glenda Jackson, another former minister, said: "Tony Blair is relying on Tory votes to force through his legislative programme. He is becoming the Prime Minister of a minority administration." John McDonnell, who chairs the left-wing Campaign Group of Labour MPs, said: "Not for 80 years, since Ramsay Macdonald, have we been in a situation where a Labour prime minister has been willing to rule on the basis of Tory votes."
John Denham, the former Labour Home Office minister, said there was virtually no information to back the police's request for a 90-day limit. He said: "Had the police been told or asked to produce a much more substantive analysis of why it should be this particular length of time, everybody would be in a much better position."
Downing Street cited the seven-page paper produced last month by Andy Hayman, the Assistant Commissioner of the Metropolitan Police, as the justification for the move. But a Tory spokesman said last night: "It is an extremely weak document. The logic of it is that there should be no limit at all."
Sir Ian Blair, the Metropolitan Police Commissioner, admitted there was "no magic" about the 90-day limit but said it was based on "our professional opinion" about the "chilling" evidence available. He told Westminster journalists: "I do accept that what we are putting forward is unknown in peacetime, a fundamental derogation from the normal judicial processes of the UK.
"But at the same time, I have been a cop for 30 years, I have been a top cop for 10. I have never seen anything like what is happening at the moment. There are people in this country plotting mass atrocity without warning."
Sir Ian said: "There is no magic. We are not saying 90 versus 89. What we are saying is 90 vs 60 or 90 vs 30. We are at 90 and we are sticking at 90 because in our experience of a number of trials, a number of investigations, that is the time it is taking to get a coherent picture of what is in front of us."
An overwhelming majority of Labour members who responded to a survey by Compass, a left-wing pressure group, rejected the 90-day proposal. Forty-six per cent wanted the current 14-day limit to be retained, 36 per cent thought it should be raised to 24 days and only 13 per cent supported a 90-day limit.