Counter-terrorism experts are also concerned that the threat is no longer only being imported from abroad, as Britain becomes a breeding and training ground for the extremists.

The warning was made by Peter Clarke, Deputy Assistant Commissioner of the Metropolitan Police, head of Scotland Yard’s anti-terrorist branch and national co-ordinator of terror investigations, in an interview that is due to be transmitted in a BBC television programme tomorrow.

Asked how many Muslims in Britain he was looking at in the belief that they may be involved directly or indirectly in terrorism, Mr Clarke said: “All I can say is that our knowledge is increasing and, certainly in terms of broad description, the numbers of people who we have to be interested in are into the thousands.

“That includes a whole range of people, not just terrorists, not just attackers, but the people who might be tempted to support or encourage or to assist.”

Mr Clarke, interviewed for BBC Two’s Al-Qaeda: Time to Talk?, added: “What we’ve learnt since 9/11 is that the threat is not something that’s simply coming from overseas. What we’ve seen all too graphically and all too murderously is that we have a threat which is being generated here within the United Kingdom.”

The figures provided by Mr Clarke are the highest that any investigator has been prepared to reveal in public, and indicate the pressure faced by police and MI5.

His estimates of possible suspects include not only activists prepared to carry out attacks but the extremist recruiters who find and encourage them and the supporters who house them, raise cash and help them.

Previous estimates have suggested a sharp growth in the numbers of people in Britain who are prepared to involve themselves in terrorism.

According to a report by the Intelligence and Security Committee published this summer, MI5 was aware of about 250 possible suspects at the time of the September 11 attacks in the United States in 2001. By July 2004 the number of “primary investigative targets” of which the service was aware had doubled to 500.

The report into the July 7 bombings said that a year later, at the time of the London bombings, the number had increased again, to about 800. Other terrorism experts have put the current figure at more than 1,000.

Before the first anniversary of the London bombings, Mr Clarke said that Scotland Yard was running more counter- terrorism investigations than ever before.

Senior officers have said that the pressure has increased dramatically. Between September 2001 and June 30 this year, a total of 1,047 people were arrested for offences committed under the Terrorism Act 2000. Police have charged 174 with non-terrorist offences, 69 have been detained and dealt with by the Immigration Service and 158 have been charged with Terrorism Act offences. About 70 are awaiting trial for terrorist charges and the number of convictions on terrorist charges remains fewer than 20.

But detectives say that other suspects have been convicted for criminal offences or immigration breaches.

According to Scotland Yard, there are about 70 “live” investigations under way, and Whitehall officials have said that MI5’s counter-terrorism workload is also at unprecedented levels.

Speaking on the anniversary of the July 7 attacks, Sir Ian Blair, the Commissioner of the Metropolitan Police, said that the security position was “very grim”.

He added that in the past year the threat had palpably increased, and he feared that we would have to accept now, and for some years to come, that we were living in an age when the threat of an attack getting through was very real.

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