Some 30 major acts of terrorism are being planned in Britain and future threats could involve chemicals and nuclear devices, the head of the country's intelligence agency said.
Eliza Manningham-Buller, director general of MI5, said young British Muslims were being groomed to become suicide bombers and that her agents were tracking some 1,600 suspects, most of whom were British-born and linked to Al-Qaeda in Pakistan.
"We are aware of numerous plots to kill people and damage our economy. What do I mean by numerous? Five? Ten? No, nearer 30 ... that we know of," Manningham-Buller said in a speech in London on Thursday evening.
She said her warnings were not intended to alarm but to paint a frank picture of the Al-Qaeda threat, which she described as sustained and growing.
Britain suffered its worst peacetime attack in July last year when four British Muslims blew themselves up on London's transport network, killing 52 commuters and wounding hundreds.
Anti-terrorist police have repeatedly warned that the terrorist threat has increased significantly since the July 7 attacks and say they have thwarted at least five major plots since then.
In August, police said they had foiled a plot to blow up transatlantic airliners using liquid explosives.
Manningham-Buller, who rarely speaks in public, provided the most detailed account of how serious that threat was.
"My officers and the police are working to contend with some 200 groupings or networks, totaling over 1,600 identified individuals who are actively engaged in plotting or facilitating terrorist acts here and overseas," she said.
Manningham-Buller said the number of cases being pursued by security services had risen by 80 per cent since January.
"Today we see the use of home-made improvised explosive devices. Tomorrow's threat may -- and I suggest will -- include the use of chemicals, bacteriological agents, radioactive materials and even nuclear technology," she said.
Earlier this week Dhiren Barot, a British Muslim convert, was jailed for 40 years for planning to blow up the New York Stock Exchange and carry out attacks in Britain using a "dirty bomb" and gas-filled limousines.
Manningham-Buller said it was clear from so-called "martyrdom" videos that suicide bombers were motivated in part by "their interpretation as anti-Muslim of UK foreign policy, in particular the UK's involvement in Iraq and Afghanistan".
A growing number of people are moving "from passive sympathy toward active terrorism" through being radicalized or indoctrinated by friends, families or organized training events in Britain and abroad, Manningham-Buller said.
She said television and Internet chatrooms had helped to attract greater numbers of recruits, and warned that the threat would last for a generation.
"It is the youth who are being actively targeted, groomed, radicalized and set on a path that frighteningly quickly could end in their involvement in mass murder of their fellow UK citizens," she said.
"Chillingly, we see the results here. Young teenagers groomed to be suicide bombers," she said. The text of her speech was later published on the MI5 Web site.