THE 28-day detention limit for terrorist suspects should be extended, Britain's most senior police officer told MPs yesterday as he warned of a "mounting" number of plots in the UK.
Sir Ian Blair, the Scotland Yard Commissioner, said a pre-charge period "somewhere between" 50 and 90 days was needed.
Giving evidence to the home affairs select committee, Sir Ian said: "The number of the conspiracies, the number of conspirators within those conspiracies and the magnitude of the ambition in terms of destruction and loss of life is increasing year by year." He insisted that although police had not yet required more than four weeks to assemble evidence, "we believe that case will emerge".
Sir Ian added: "At some stage, 28 days is not going to be sufficient, and the worst time to debate whether an extension is needed would be in the aftermath of an atrocity."
Government plans to extend the limit to 90 days were previously rejected by the Commons - although MPs did vote to double the period from 14 days in 2005. However, ministers have said they want to re-examine the situation as police investigations become increasingly complex.
Peter Clarke, the Metropolitan Police's anti-terrorism chief, also backed an extension, citing an example from a current high-profile investigation - which he refused to identify.
He said detectives had used a chemical treatment on the interior of a garage, which needed two weeks to develop but revealed a handprint high up on a wall. Police then searched the roof space and located a portable computer storage device which contained vital evidence.
Mr Clarke said that terror investigations were not a "cavalry charge", and "throwing resources" at them would not always speed up the process.
Sir Ian said strong judicial scrutiny of detention was essential, but he rejected the claim that there should be no upper limit and the matter should be left to the discretion of the courts.
"If you leave it wide open in terms of the length of time, you do leave a situation which can be represented as detention in perpetuity," he told MPs.
Lord Carlile, the UK government's independent reviewer of terrorism legislation, said he believed there would be a few cases where detention beyond 28 days would be useful.
Although none has yet appeared, there would probably be two or three examples over the next five years, and that supported calls for an extension.
"In my view, in the interests of national security it would be justifiable," he told MPs.
However, David Davis, the shadow home secretary, said: "We have the longest period of detention without trial in the democratic world.
"Instead of more Draconian legislation, there are immediate practical measures the government should be taking that would make Britain safer, including the introduction of a real border police force, post-charge questioning of terror suspects and lifting the ban on using intercept evidence to prosecute terrorists."
Sir Ian accepted that any move to lengthen detention periods would not "ease" relations with the Muslim community.