Britain's spy chief warned on Friday of nearly 30 terror plots under investigation, a revelation which Prime Minister Tony Blair said underlined the gravity of the threats facing the country.
Dame Eliza Manningham-Buller, head of the MI5 domestic intelligence agency, disclosed that her agents were tracking over 1,600 suspects from 200 groups, most with ties to Osama bin Laden's al Qaeda terror network.
She also warned that while home-made, improvised explosives may be used now, future terror attacks could involve chemical, bacteriological, radioactive and even nuclear material.
In a rare sortie from the shadows, Manningham-Buller said MI5's caseload of British-based terror sympathisers -- many of them British citizens -- had increased by 80 percent since January.
The "scale and speed" of radicalisation were among the most alarming aspects of the problem, she said.
"What we see at the extreme end of the spectrum are resilient networks, some directed from al Qaeda in Pakistan, some more loosely inspired by it, planning attacks including mass casualty suicide attacks in the UK," she accused.
"Martyrdom" videos of suicide bombers were motivated in part by their interpretation of British foreign policy, particularly in Iraq and Afghanistan, as "anti-Muslim", she said in a closed-door speech to academics in east London on Thursday, but which was made public on MI5's website on Friday.
"My officers and the police are working to contend with some 200 groupings or networks, totalling over 1,600 identified individuals -- and there will be many more we don't know -- who are actively engaged in plotting, or facilitating, terrorist acts here and overseas," she said.
Britain has been on high alert since the July 7, 2005 bombings on London's public transport network killed 52 commuters and the four extremist suicide bombers.
The MI5 chief said that since the attacks, security forces had thwarted "a further five major conspiracies in the UK, saving many hundreds (possibly even thousands) of lives."
She also expressed concern that many of those involved were young men and teenagers as young as 16.
"It is the youth who are being actively targeted, groomed, radicalised and set on a path that frighteningly quickly could end in their involvement in mass murder of their fellow citizens or their early death in a suicide attack or on a foreign battlefield," she said.
Blair backed the warning from Manningham-Buller that the threat of terrorism was here to stay.
"I've been saying for several years that this terrorist threat is very real; it's been building up over a long period of time," he said.
"This is a threat that has grown up over a generation. I think she is absolutely right in saying that it will last a generation."
He said the terror threat could only be tackled by toughening up security laws and by combating the "poisonous propaganda" of extremists out to "warp and pervert minds".
"It's a very long and deep struggle this, here and right round the world, but we've got to stand up and be counted," Blair added.
Manningham-Buller's assessment came after Muslim convert Dhiren Barot was jailed for life this week for plotting to kill thousands of people in devastating attacks in Britain and the United States.
Some experts believe the security forces could struggle to keep pace with the growing number of people being radicalised.
MI5 has 2,800 staff but by 2008, it will be twice the size it was before the September 2001 attacks on the United States.
Professor Paul Wilkinson, of the Centre for the Study of Terrorism and Political Violence at St Andrews University, said the police and security services were "stretched to near breaking point".
"I do not think she is hyping the problem at all. It squares with what we know from recent trials and police statements," he said.
"It takes about 20 officers to provide really close, in-depth surveillance on just one person.
"You can imagine the strain on resources if you take that and multiply it by the numbers she quoted."