Over the last five years, much of the command, control and inspiration for attack planning in the UK has derived from Al Qaeda's remaining core leadership in the tribal areas of Pakistan, according to the head of Britain's intelligence service MI5.
In a rare speech to the Society of Editors' annual conference in Manchester Monday titled 'Intelligence counter terrorism and trust', Jonathan Evans, the MI5 chief, said that often the planning used young British citizens to mount the actual attack.
The earlier MI5 chief, Eliza Manningham-Buller, had pointed out last year that the service had identified 1600 individuals who it believed posed a direct threat to national security and public safety. The number, Evans disclosed, had now risen to 2000.
He said: "As I speak, terrorists are methodically and intentionally targeting young people and children in this country. They are radicalising, indoctrinating and grooming young, vulnerable people to carry out acts of terrorism.
"This year, we have seen individuals as young as 15 and 16 implicated in terrorist-related activity...Over the last five years much of the command, control and inspiration for attack planning in the UK has derived from Al Qaeda's remaining core leadership in the tribal areas of Pakistan - often using young British citizens to mount the actual attack".
Evans singled out Al Qaeda and its associated groups as the "main national security threat" faced by Britain. Al Qaeda, he said, was conducting a deliberate campaign against Britain.
He said: "It is the expression of a hostility towards the UK which existed long before Sept 11 2001. It is evident in the wills and letters left behind by actual and would-be bombers. And it regularly forms part of Al Qaeda's broadcast messages.
"Another development in the last 12 months has been the extent to which the conspiracies here are being driven from an increasing range of overseas countries. But worryingly, we have more recently seen similar processes emerging elsewhere.
"This sort of extension of the Al Qaeda brand to new parts of the Middle East and beyond poses a further threat to us in this country because it provides Al Qaeda with access to new centres of support which it can motivate and exploit, including in its campaign against the UK".
Touching upon several sensitive issues, Evans went on to reveal that this year there were "high levels of covert activity by foreign intelligence organisations in our country".
He said: "(Despite) the Cold War ending nearly two decades ago, my Service is still expending resources to defend the UK against unreconstructed attempts by Russia, China and others, to spy on us.
"A number of countries continue to devote considerable time and energy trying to steal our sensitive technology on civilian and military projects, and trying to obtain political and economic intelligence at our expense.
"They do not only use traditional methods to c collect intelligence but increasingly deploy sophisticated technical attacks, using the internet to penetrate computer networks.
"It is a matter of some disappointment to me that I still have to devote significant amounts of equipment, money and staff to countering this threat. They are resources which I would far rather devote to countering the threat from international terrorism - a threat to the whole international community".
The theme of the annual conference of the Society of Editors is 'A Matter of Trust'.