The head of MI5 has warned the agency is dealing with up to 30 alleged "mass casualty" terror plots in the UK and abroad involving more than 1,600 individuals.
Dame Eliza Manningham-Buller said MI5 and the police are tackling 200 groups or networks totalling more than 1,600 identified individuals in the UK who are "actively engaged in plotting or facilitating terrorist acts".
She said the terror plots involved plans for suicide attacks with some being directed by al-Qaeda.
Dame Eliza's assessment came in her first public speech since the July 7 terror attacks last year, to an invited audience of academics in east London.
She warned of the prospect that weapons of mass destruction could be used in future terror attacks in the UK.
Dame Eliza said: "Today we see the use of home-made improvised explosive devices, but I suggest tomorrow's threat will include the use of chemicals, bacteriological agents, radioactive materials and even nuclear technology."
She told her audience that MI5's caseload of UK-based terror sympathisers - many of them British citizens - had increased by 80 per cent since January.