Terrorists may well have links to al Qaida but it is not the powerful SMERSH-type organisation which politicians would have us believe, according to a Birmingham academic.

Toby McLeod, lecturer in war studies at the University of Birmingham, said al Qaida was a shared philosophy rather than an organised network similar to the one featuring in the James Bond novels and films, with a timetable of terror for Britain's cities.

He said membership was loose and changing and 'authority' was represented by a handful of clan and religious heads in the North West Frontier province in Pakistan.

"We in the West tend to view what is called al Qaida through the lens of our own culture," he said.

"In the West we get the impression that it is like SMERSH with an office headquarters over several floors and a complicated hierarchy. But that's the problem with our guys in intelligence - they've grown up with the Cold War and John Le Carre and still think of an enemy like that."

He said the idea of al Qaida took its inspiration from two sources - the Muslim brotherhood in Egypt, and Wahabism, the puritanical strand of Islam from Saudi Arabia, both of which had been around for at least 50 years.

They shared a methodology which worked from the bottom up, he said.

"A group of fanatics will dream up a scheme, and then get financial support or a seal of approval for it, rather than it coming from the top down with a boardroom planning horrific acts," he said.

Approval if sought would generally be via word of mouth, and second or third-hand.

"It would be via two or three intermediaries, that so and so is happy with what you are doing," he said. "It is not formalised communication and it is difficult to find out who does what. These people will be consulted because of their tribal or religious standing, and links will be through extended family and clan.

"People in Birmingham may have these links, because we have a big Pushtu community who have family there. It doesn't make someone a wild-eyed terrorist because so-and-so is related to so-and-so who is related to so-and-so and he knows someone. It is the classic six degrees of separation."

Given the indefinable nature of al Qaida, he said it was next to impossible to attribute plots such as that by Moinul Abedin, jailed for 20 years in 2002 after being convicted at Birmingham Crown Court of intent to cause an explosion.

Mr McLeod said he had particular doubts about 'retrospective' analysis of earlier plots.

"It's all hearsay and supposition; how do you demonstrate it?" he said. "I think it is a convenient label.

"Reid has got to play to different audiences. He's got to play to right-wing tabloid readers who are interested primarily in personal safety.

"He can't say, 'well we believe this but we are not sure', because he will sound incompetent, particularly hard on the heels of what happened to Charles Clarke."