It is, as she acknowledged, rare for the head of MI5, the Security Service, to speak out in public. What Dame Eliza Manningham-Buller said yesterday on the threat from al-Qaeda in Britain also had to wait until after the trial of Dhiren Barot. But her assessment is as compelling as it is alarming. Since the suicide attacks in London, M15 has thwarted five major conspiracies in Britain. Last month a total of 99 defendants were awaiting trial in 34 cases. The Security Service is investigating about 200 extremist groups or networks, totalling 1,600 individuals — as well as many who are unknown — who are believed to be engaged in plotting terrorist acts here and abroad.

The extremists are motivated by a sense of grievance and injustice driven by their interpretation of the history between the West and the Muslim world. Polls say that up to 100,000 British citizens consider that the London attacks were justified. More people are moving from passive sympathy towards active terrorism, radicalised by friends, families, in organised training events, by images on television, and in chat rooms and websites on the internet. And al-Qaeda is quick to take advantage, admitting that half its war is waged through the media.

How to confront this unprece- dented challenge to security must now be the priority not just for MI5 and the Government, but for employers, schools, faith leaders and the Muslim community itself. The Security Service has had to move fast. Its caseload has gone up 80 per cent since January. A 2,800-strong workforce has had an increase of almost 50 per cent since 9/11, a quarter of them under 30 — but only 6 per cent from ethnic minorities, a figure which will need to rise substantially given the need for special language abilities. The budget may need further increases. But recent proposals to create a more streamlined anti-terror organisation, similar to the US Department of Homeland Security, will help.

As important, however, is a better understanding of the causes of extremism. Dame Eliza mentioned a few, and rightly underlined that British foreign policy in Iraq and Afghanistan is indeed, despite official attempts to play this down, seen by many as anti-Muslim. She also identified the pernicious influence of a few preachers and people of influence promoting an extreme and minority interpretation of Islam. But other factors must be acknowledged. One is the heterogeneous nature of Islam in Britain, where adherents come from many different countries, traditions, sects and ethnic groups. In the absence of any religious hierarchy, primacy is achieved by a national profile that depends on how zealously and noisily Islamic credentials can be established. There are no rewards for moderation, seen often as compromise with the Establishment and secular society.

Another clear motivation is the alienation of young Muslims — low educational and economic achievement, a sense of exclusion and a growing generation gap. But the recent torrent of criticism of Islam, some of it provocative and distorted, has heightened fears and a sense of persecution. That must not mean silence on difficult issues of public concern, such as Jack Straw’s remarks on the veil. It does make more urgent the need for constant engagement, at all levels, in building bridges, seeking shared values and countering extremism. Otherwise, as Dame Eliza said, al-Qaeda will recruit here for years to come.