A meeting of the Shadow Cabinet decided to hold firm to its line that 28 days should be the maximum period of detention under the Terrorism Bill, in spite of concerns among Conservative MPs that they should not be going against the wishes of the police.

The Tory whips were claiming last night that they had hardened the resolve of doubtful MPs by emphasising that giving way would allow Tony Blair a decisive political victory. If the Conservatives and other Opposition parties hold firm on the crucial vote today, Mr Blair will still face the prospect of defeat on the 60-day detention proposal — unless he has succeeded in heading off enough of his own rebels on the issue.

If the Government loses over the 90-day plan MPs will then vote on a proposal to set the detention without charge limit at 60 days. If that vote is lost it is almost certain that an amendment tabled by a Labour MP for a 28-day period will pass.

Sir Ian Blair, the Metropolitan Police Commissioner, yesterday intensified the police case for a much longer detention period. He accepted there was "no magic" around the 90-day plan. "It is our opinion, looking at the investigations that we have done, that we would have been in a much, much better state to interview people around what we have found after about three months than we were inside 14 days."

"We are sticking at 90 because our experience in a number of trials, a number of investigations is this is the length of time it is taking to actually get a coherent picture of what we have in front of us."

He said that after the July 7 attacks it took eight weeks to get the position under control and analysed. "Three months is the kind of time you would need to put those questions to people after that."

Sir Ian added that he accepted that the police police proposals were unknown in peacetime. "I have been in the police service 30 years. I have been a top cop for rten years. I have never seen anything like what is happening at the moment. There are people out there in this country plotting mass atrocities without warning. Under these circumstances we believe that the State has a duty on behalf of its citizens to give the greatest level of protection it possibly can.

"This is different. It is chilling, and we are very, very worried and alarmed about it."

Gordon Brown threw his weight publicly behind Mr Blair. He said: "When you have advice from the Association of Chief Police Officers, the head of the Metropolitan Police and those specialists concerned in the detailed work of interviewing and dealing with terrorist suspects, and they are all saying the same thing about the need for the extra time, then you ought to take that very seriously." Mr Brown added: "I am surprised and shocked at the short-term opportunism of our opponents in the Conservative Party, particularly the two leadership candidates in the party who wish to put short-term political opportunism before the long-term strategic interest of our country."

Police chiefs have begun an unprecedented national campaign to lobby MPs in support of 90-day detention.

Chief Constables have been told to contact their MPs to brief them and write to their local newspapers putting the case for the case for the clause, The Times has learnt. Last week Sir Ian Blair, Andy Hayman, head of Scotland Yard’s counterterrorist units, and Mike Todd, the Chief Constable of Greater Manchester Police, wrote in national newspapers.

At the weekend, other chief constables began contacting MPs, especially those with doubts about the legislation, and Mr Hayman appeared on national television and radio. Senior officers say that they are defending a proposal which they put forward themselves and deny being pressured to act by the Government.

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