In an interview recorded in July, before the arrest of more than two dozen people on suspicion of an alleged plot to blow up US-bound aircraft, Peter Clarke said Britain was facing an increasing threat from homegrown extremists.

He refused to give an exact figure about how many people he said were either directly or indirectly involved in plotting atrocities on British soil but said the number ran into four figures.

"I don't want to go down the numbers game, I don't think it's helpful," he told a BBC programme entitled "Al-Qaida -- Time to Talk?", to be shown Sunday evening.

"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."