A resurgent al Qaeda has made the UK its priority target and is building active service units among disaffected young British Muslims, according to senior intelligence officials.
MI5 also fears the July 7 Tube and bus bombings were “just the beginning” of a rolling campaign aimed at inflicting mass civilian casualties and damaging the economy.
The terror network has also taken a lead from the Provisional IRA, organising followers into self-contained cells to make infiltration by the security services almost impossible.
They believe each cell has a leader, a quartermaster responsible for obtaining bomb-making materials and weapons, and a small number of volunteers drawn mainly from ethnic Pakistani communities concentrated in London, the West Midlands and even Glasgow.
The fact that British citizens make 400,000 trips to and from Pakistan each year to visit family is being used by a radicalised minority to connect with al Qaeda contacts.