Must we really turn our city into a fortress?

Yesterday, the Prime Minister announced another security crackdown. Last week, Jonathan Evans, the head of MI5, warned of 2,000 would-be terrorists in Britain. Soon, the Government will try again to extend detention without charge. It feels like a good time to consider the reliability of official statements about the terrorist threat.

Let's be clear about one thing: this is no new Iraq. This threat is real. Evans wasn't lying. But he might be spinning, just a bit.

What he did in his address was refer to a speech by his predecessor, Eliza Manningham-Buller, saying MI5 had identified, in Evans's words, "around 1,600 individuals who we believed posed a direct threat to national security and public safety because of their support for terrorism". This figure, Evans added, had now risen to 2,000.

But what Manningham-Buller actually said last year was not quite that simple. She said that the 1,600 figure comprised people who were "plotting, or facilitating, terrorist acts, here and overseas."

Unthinking media reporting of these speeches gave the impression of 1,600 or 2,000 actual terrorists ready to attack us. But that's not what they said. The figures include those who merely "facilitate" (Manningham-Buller) or "support" (Evans).

A "facilitator" could be someone who would never themselves kill, but who might host a radical website on their computer or even just donate money. A "supporter" is even broader.

Then there's Manningham-Buller's vital rider that the figure includes those plotting or facilitating attacks " overseas". That could, and probably does, include groups completely unrelated to al Qaeda, such as the Tamil Tigers, who raise money in Britain but are absolutely no threat to us here.

Most importantly, there's that careful term, a terrorist "act". This need not be an attack. It is anything the authorities define as terrorism - and that's now a pretty wide definition, as we saw last week with the conviction of 23-year-old Samina Malik for writing poems celebrating beheading.

In law, that made her a terrorist. But even the prosecution admitted Malik had no contact with anyone else or any intention of acting on her post-adolescent fantasies.

The Government reminds us that this year, 32 people have been convicted of terrorist offences. But if there really are 2,000 "plotters", why have so few of them been brought to justice? And several of those 32 have been "thought criminals" like Samina Malik - undesirable and wrong, certainly; deserving of punishment, possibly; but a direct threat to life and limb? I think not.

Even in the more serious plots that have come to court, the evidence, for anyone paying attention, reveals (broadly) inexperienced, amateurish terrorists with only the most tenuous links to wider networks. The sophistication of the 9/11 attacks, two years in the planning, is gone for ever. It would now be detected.

The truth is that the terrorists certainly want to kill us; they have done so in the past; and they will certainly do so again. But they cannot do it very often.

In the more than six years since 9/11, a time of unprecedented Western-Muslim tension, there have been just two "al Qaeda" attacks causing loss of innocent life in the entire Western world. In Britain, hospital superbugs kill more of us every four days than al Qaeda has managed in its entire existence.

None of this is to argue that the terrorist threat to this country is not serious. It is. In London, we know this better than anyone. But it needs to be kept in proportion; and it needs, above all, to be understood.

We need to understand that terrorism's main purpose is not to kill. It is to terrorise. It's not what they can do to us. It's what they can persuade us to do to ourselves.

We run a very low risk of being caught in an attack. But on current form, we are running a very high risk indeed of attacking our free society in ways that harm us all, and benefit only the extremists.

That is why the Government is getting its anti-terror strategy so badly wrong. To propose, yesterday, airport-style security at mainline railway termini is not only pointless (unless they seal off every one of the 2,500 other stations, too, there's nothing to stop anyone getting on at, say, Wandsworth Common and bringing his bomb into Victoria by train).

It will not only harm the economy, restricting commuter flow and scaring off visitors. It, and the proposed roadblocks and security barriers, will also start a process of Belfastisation, symbolically changing our free and normal city into a fortress.

Ordinary things, like catching a train, will suddenly become more complicated, particularly if you have brown skin. That'll make Muslim kids happy to be British, all right.

Even more counterproductive, of course, is Gordon Brown's 56-day detention proposal, grossly disproportionate to the threat and unnecessary by the admissions of about five ministers now.

With or without "56 days", further attacks, everyone agrees, are inevitable. But though we cannot refuse to be attacked, we can refuse to be terrorised.

Instead of frightening physical defences all over London, what we should be building is defences, in our minds, against fear.

What defeats fear is information, and we need more of it. Not enough of us know, for instance, that it is in fact extremely difficult for amateurs to make a very effective chemical weapon.

Instead of ill-explained numbers with noughts on the end, what we need from ministers and security chiefs is education: a calm discussion of the terrorists' strengths - and weaknesses. Foiled plots should be presented as evidence of terrorist failure, not proof of an evergreater risk of terrorist success.

Of course, if you are a government trying to ram through a macho, misguided security agenda, public ignorance and fear suits you rather well. Unfortunately, it also suits the terrorists.