There are up to 30 alleged "mass casualty" terror plots in operation in Britain, as well as hundreds of young British Muslims on a path to radicalisation, the head of MI5 has said.

In an unprecedented public announcement yesterday, the MI5 director general, Dame Eliza Manningham-Buller, revealed that the caseload of the Security Services had risen by 80 per cent since January, and that the counter-terrorism agency was fighting to keep the rapidly growing threat under control

Describing the scale of the home-grown terrorist problem, she said MI5 and the police were tackling 200 groups or networks totalling more than 1,600 identified individuals in the UK who were "actively engaged in plotting or facilitating terrorist acts".

Islamic militants linked to al -Qa'ida were recruiting teenagers to carry out attacks using chemicals, bacteriological agents, radioactive materials and even nuclear technology.