Al Qaeda has "sleepers" in the United Kingdom who live normal lives but are ready for terrorist action, the head of MI5 warned last night.

Director general Eliza Manningham-Buller said: "Western security services have uncovered networks of individuals, sympathetic to the aims of al Qaeda, that blend into society, individuals who live normal, routine lives until called upon for specific tasks by another part of the network."

She warned that the threat from terrorist extremists will remain for a "long time" as a result.