Tony Blair has given his backing to the head of MI5 for her unprecedented public warning about the scale of the terrorist challenge facing UK security agencies.

Dame Eliza Manningham-Buller, the MI5 Director-General, said yesterday that hundreds of young British Muslims are being radicalised, groomed and set on a path to mass murder.

She also revealed that the Security Service’s caseload had risen by 80 per cent since January and now involved about 30 "Priority 1" plots. It has identified 200 terrorist networks involving at least 1,600 people, many under the direct control of al-Qaeda leaders in Pakistan.

"More and more people are moving from passive sympathy towards active terrorism through being radicalised or indoctrinated by friends, families, in organised training events here and overseas," she said. "Young teenagers are being groomed to be suicide bombers."

Reacting to her comments, the Prime Minister said today: "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.

"This is a threat that has grown up over a generation. I think she (Dame Eliza) is absolutely right in saying that it will last a generation."

Speaking at No 10 Downing Street after a meeting with Helen Clark, his New Zealand counterpart, Mr Blair described terror as a global problem that should be tackled both by tougher laws and by countering the propaganda of those who "warp and pervert" the minds of young people.

"The values that we have and hold dear in this country - that are about democracy, tolerance, liberty and respect for people of other faiths - are the values that will defeat those values of hatred and division and sectarianism," he said.

"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 for what we believe in."

In an address to an audience from the Mile End Group run by Peter Hennessy, Professor of Contemporary British History at Queen Mary, University of London, Dame Eliza said that she was alarmed by the "scale and speed" of the radicalisation. Security sources said later that process had intensified since the 7/7 bombings.

"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.

"Killing oneself and others in response is an attractive option for some citizens of this country and others around the world. [The] threat is serious, is growing and will, I believe, be with us for a generation. It is a sustained campaign, not a series of isolated incidents. It aims to wear down our will to resist."

Dame Eliza admitted that, despite a major recruitment drive, just 6 per cent of MI5’s staff came from ethnic minorities. This compares with 8 per cent in the Metropolitan Police. Security sources insisted that change was happening and, of 400 people recruited this year, 14 per cent were from ethnic minority groups.

The MI5 chief timed her stark assessment to coincide with the conviction of Dhiren Barot, the al-Qaeda planner who was jailed for 40 years this week for plotting car bomb and dirty bombs attacks in London.

In her speech she said the methods used by terrorists had become more sophisticated.

"Today we see the use of home-made improvised explosive devices," she said. "Tomorrow’s threat may, and I suggest will, include the use of chemical, bacteriological agents, radioactive materials and even nuclear technology," she said.

Her assessment of 30 Priority 1 plots is a significant increase on the 24 "major conspiracies" referred to by John Reid, the Home Secretary, in August.

Both police and security sources have given warning that Britain has become the No 1 target for al-Qaeda.

The significant Muslim population and the constant flow of British-born Pakistanis visiting their families in Pakistan every year have been cited as providing al-Qaeda with opportunities for converting young people to terrorism.

"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 we don’t know) who are actively engaged in plotting or facilitating terrorist acts here and overseas," Dame Eliza said.

Those terror networks "often have links back to al-Qaeda in Pakistan and, through those links, al-Qaeda gives guidance and training to its largely British foot soldiers here on an extensive and growing scale".

The head of MI5’s speech, which was approved by ministers, comes after recent warnings given by Peter Clarke, head of Scotland Yard’s Counter Terrorism Command, and by Mr Reid. They both said that the terrorist threat would be long and enduring.

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