In a report published today, Prof Clarke says that, over the next five to 10 years, about 800 prisoners – in jail for non-terrorism offences – are due to be released on to the streets having been radicalised in jail.
They will be joined by convicted terrorists serving short sentences who, once freed, are likely to be just as committed to the cause of jihad as before they were jailed, the report claims.
Prof Clarke, who advised Gordon Brown as a member of the National Security Forum and is a visiting professor at King's College London, warns that this "new wave" will pose a significant challenge to the security services responsible for identifying and monitoring them.
While previous al-Qaeda tactics involved so-called "spectacular" attacks, the report warns that the terrorist group's leaders, such as Yemeni preacher and US citizen Anwar al-Awlaki, are encouraging individuals to launch less sophisticated but equally deadly attacks on crowded places.
Their targets have also changed from concentrating on aircraft to including attacks on trains, hotels and sporting events. The report will serve as a stark reminder to the Government and public that the threat from Islamist terrorism remains severe, even though there has not been a fatal attack on British soil since 2005.
The current government threat level stands at "severe", indicating a terrorist attack is considered "highly likely". The level was raised from "substantial" in January.
In the Western world, Britain has the "greatest to fear" from home grown terrorists, the report says.
One of the major threats in Britain, according to Prof Clarke, is from released prisoners who may have been convicted of terrorist offences or may have been radicalised while in jail. "British prisons still house more terrorists than in any other European country, though not for very long periods," he warns.
He points out that just 23 people, around 19 per cent of those convicted of terrorism offences, have been given life or indeterminate sentences. Twenty per cent have been sentenced to more than 10 years, and the largest single proportion, 32 per cent, received between eight months and four years. "It raises immediate questions about the motivations of those now released, or soon to be released: are they more or less inclined to reoffend?" he says.
"From previous experience in Northern Ireland, it is more likely that the majority of those released will remain as committed to their cause as before, and may serve as a source of motivation to others, albeit in clandestine ways."
Prison authorities have become increasingly concerned about radicalisation behind bars, especially in the eight high-security jails where most terrorist prisoners are kept.
Probation officers have warned that about one in 10 of the 8,000 Muslim prisoners in high-security institutions in England and Wales is successfully targeted.
This amounts to "around 800 potentially violent radicals, not previously guilty of terrorism charges, [who] will be back in society over the coming five to 10 years," Prof Clarke says.
These radicals are ideal candidates to form a "new wave" of terrorists threatening Britain, the report says.
The release of 800 prisoners would see an increase by nearly a half of the 2,000 radicalised individuals MI5 is currently said to be watching.
Large, well co-ordinated terrorist attacks have become more difficult to carry out and instead attacks have evolved into “more individual efforts” warns the report by Prof Clarke and co-author Valentina Soria in the Royal United Services Institute Journal.
They point to attacks such as that of Umar Farouk Abulmutallab, a former student in London, who tried to blow himself up in an aircraft coming into land in Detroit on Christmas Day last year and also the Times Square attack by Faisal Shahzad in May.
“Rather than sending out trained 'cell leaders’ to conduct preparation for sophisticated operations, AQAP(al-Qaeda in the Arabian Peninsula) and other related organisations have recently been content to send out a higher number of lone individuals (or at least lightly supported ones) whose chances of success are considerably lower but whose number and presence raise similar public anxieties,” the report says.
“Eventually, it is reasoned, one of them will be lucky enough to succeed in a major way against high profile targets in western countries.”
Britain’s “globalised society” makes it more vulnerable, says Prof Clarke. “In an open society there is only so much that any government can do to protect the public.’’
