Dame Eliza Manningham-Buller was appointed head of MI5, the Security Service, in 2002. In former years nobody in the Security Service could be named in the media. Nowadays, following the American example of the director of the CIA being known by name in public, we — the public, and we pay the bill — are allowed to know just the head boy or girl.

Dame Eliza, as the press in Britain now call her, was born into the British aristocracy in 1948. Her father was the 1st Viscount Dilhorne, a former Conservative lord chancellor, and he sent her to a private school, Benenden. She went on to Lady Margaret Hall, Oxford, and was recruited by MI5 in 1974. She became deputy director-general in 1997. One gets the impression it would be unwise to mess her about; as her name suggests, she is a figure of considerable authority.

On Nov. 9 she gave an address to a carefully chosen audience in London. The meeting had been arranged by professor Peter Hennessy from the University of London. Her message was highly alarming, but she, herself, came over as a confident person, who knew her facts and also knew what needed to be done. Her subject was the unpleasant one of international terrorism and its threat to the United Kingdom — the new scourge of our time.

Just before the invasion of Iraq the government gave the impression that it was exaggerating the security case, to reinforce their case for invading Iraq with the Americans. Enough has been revealed since 2003 to know there was truth in this accusation. On one occasion Scimitar armored vehicles were deployed to Heathrow Airport for no good military reason: Together with their crews from the Household Cavalry they looked impressive on the 6 o’clock TV news.

The figures Dame Eliza gave were clear and striking. There are 1,600 suspects under surveillance. There are 30 “Priority 1” plots to kill or maim or seriously damage property. 200 terror networks have been identified in the United Kingdom. Most of this activity is under the direct control of Al-Qaeda leaders in Pakistan. The Security Service’s caseload has risen by 80 percent since January.

In her view:

“More and more people are moving from passive sympathy toward active terrorism through being radicalized or indoctrinated by friends, families, in organized training events here and overseas. Young teenagers are being groomed to be suicide bombers. 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 citizens, on their early death in a suicide attack or on a foreign battlefield. Killing oneself and others in response is an attractive option for some citizens of this country and others around the world.”

I find particularly chilling her belief that:

“(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.”

It is most unusual for the head of MI5 to make such a public speech; advice is normally given to ministers in private. It coincided, I suspect on purpose, with the conviction of Dhiren Barot, the Al-Qaeda planner, who was jailed for 40 years. He wanted to use stretched limousines, packed with gas cylinders, as bombs in public places.

MI5’s basic problem is well known. Its target in the 1990s was the Provisional IRA. It has had to switch quickly to young Muslim extremists, living in the United Kingdom, by no means down and out but often comparatively well educated and from good Muslim families. Just 6 percent of MI5’s staff of 2,800 comes from ethnic minorities. The Metropolitan Police figure is 8 percent. MI5 has mentioned that of the 400 people it recruited this year, 14 percent were from ethnic minorities.

Dame Eliza, unlike Prime Minister Tony Blair and Jack Straw, when he was foreign secretary, had no doubt that British foreign policy in Iraq and Afghanistan is seen by many local Muslims as being anti-Muslim. Foreign policy should be decided by what is in the national and international interest and not to appease a minority, but the government has been foolish to deny the obvious and they lost further credibility as a result.

Her speech had been cleared by ministers and followed similar warnings from Scotland Yard’s anti-terrorist experts. She has highlighted a major, complex and frightening challenge for the United Kingdom, as it attempts to understand what motivates its young citizens to embrace religious extremism. MI5 and the police have vital roles, but it is the winning of the hearts and minds of young Muslims, from the 1.6 million Islamic community, that is going to be crucial.