LONDON, England (CNN) -- The head of Britain's intelligence services has warned that children as young as 15 are becoming involved in terrorist-related activity.

Jonathan Evans, the chief of MI5, also said that at least 2,000 people are living in Britain who pose a threat to security because of the country's support for al Qaeda-inspired terrorism.

"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," he told a gathering of newspaper editors in Manchester.

Evans said the figure of 2,000 -- an increase of 400 since November 2006 -- only included those the intelligence services knew about and that the actual number could be double.

He said there had been 200 terrorist convictions in Britain since the September 11 attacks.

The MI5 head added that over recent years much of the command and inspiration for attack planning in the UK had come from al Qaeda's remaining core leadership in the tribal areas of Pakistan.

However, he said in the last 12 months terrorist plots on British soil were increasingly been inspired by al Qaeda cadres in other countries, including in Iraq and East Africa...