Yes, but what does a dirty bomb look like?

Take the ferries, I suppose we could challenge the minister to risk his own young family on 'The Pride of Bognor'?

The Prime Minister's speech to the Lord Mayor's Banquet on Monday, important though it was, was actually just a statement of the bleeding obvious. "Barely a day goes by," said Mr Blair, "without some new piece of intelligence coming via our security services about a threat to UK interests." And now I paraphrase him. "Look," he went on, "Some of this stuff is kosher, some of it is crap and we don't know which is which; some of it is so general that it's useless, some of it is more specific – but not so specific that we can predict where the next atrocity will take place. Anyway, if we knew exactly what was going to happen, then we'd stop it, wouldn't we?

"So, one day, the chances are that something nasty will happen. That's modern life. In the meantime, though, what we can't do is close everything down every time one of these warnings comes in. My message to you, therefore is just this: in the immortal words of that great wartime hero, Corporal Jones: Don't panic."

The PM's speech follows last week's mix-up, when – apparently – the wrong draft of a Home Office release was put out, warning among other things of the threat from terrorists wielding "some kind of poison gas" or "a so-called dirty bomb".

What they'd actually meant to say, it turned out, Dixon of Dock Green-like, was simply that we should watch how we went in the period leading up to Christmas. Inevitably ,one of those people who specialise in writing to the PM show on the BBC e-mailed the programme (doubtless in green font) to suggest that this withdrawn warning was really just a cunning way for the Government to soften us all up for the invasion of Iraq. Mr Blair, rather nobly I thought, seemed to be trying to put such people at ease.

They have a slightly different and more open approach in the States. Last autumn, when Tom Ridge was appointed Director of Homeland Security (a post that will soon have cabinet-level rank in the USA), a White House spokesman said that the objective was that, "when people see him, we want them to think, 'My babies are safe.' "

Since then a number of things have happened. The state of national alert has been increased from yellow to orange and reduced back to yellow again, seemingly with no connection to a whole series of warnings that have emanated from various agencies at various times. These have included the threat from "unspecified groups" who were "targeting suspension bridges on the West Coast"; a remarkably precise six rush-hour incidents were said to be possible during a six-day period, and the suggestion of further attacks on "landmarks" in New York.

Last December a warning was issued to coincide with the end of Ramadan, and on 4 July, Islamic terrorists were said to be planning to blow up nuclear power plants. The anniversary of 11 September was also flagged, and one cannot help but worry exactly when Granny bin Laden's birthday is. One of the latest FBI warnings is that al-Qa'ida may be plotting to attack passenger trains in America, "possibly using operatives who have a Western appearance". All this has prompted the actor Robin Williams to rename the CIA as the Central Intuitive Agency and has now led to widespread scepticism about any warning that comes from the centre.

None of this scepticism, however, means that nothing will happen. In Bali it already has, and I am sure there is more to come. But it does make it easy to understand Mr Blair's dilemma about what to tell us and when. After all – and this is the belated point of this article – what are we, the citizenry, supposed to DO with any information we get?

Take the ferries. For example, I mean. There seems to be a warning from the Dutch and German authorities that someone nice is planning to take a lorry packed with explosives on to a car-ferry. But the powers-that-be are not closing down the ferries, and the advice from our Government is not to change our travel plans but simply to be vigilant. Applying the old BSE test I suppose we could challenge the Home Office minister to risk his own young family on The Pride of Bognor, but you know he'd get on it anyway, smiling. So never mind Tony Blair, what do we do?

The fatalist says forget it. We can't know the method, the timing or the place. We never imagined 11 September (and if you are one of the people who did imagine it, well bully for you), and we probably won't anticipate the next atrocity either. Go about your business, whistling and happy – which in itself may make the terrorists feel intimidated. And in any case, even if you are successfully vigilant, the whole thing will just operate like an international version of Neighbourhood Watch, pushing the crime to somewhere else in the world.

Vigilant people (of whom I am one) say no. If we are all a bit more careful, a bit more watchful, a bit more knowledgeable about first aid and where the fire escapes are, then we may save ourselves and others. Did I know, asked a fellow Vigilant yesterday, that the majority of fire and disaster victims – when the positions of dead bodies have been plotted – are found to have been killed trying to pay their bills at the check-out before they flee the burning block?

But what am I looking out for? It won't, I imagine, be a guy sporting a bandanna on his head inscribed "Coming Home, Allah" in Arabic, and wearing a rather bulky T-shirt festooned with wires. Come to think of it, what does a dirty bomb (so-called, or otherwise) look like? Is it wrapped in a copy of Asian Babes or the Daily Express? What does poison gas smell like? Would you even be able to detect it during a summer's sweaty rush hour on the London Tube? As for lunatic lorry drivers, where would you even start?

There is, of course, a third position, which I infer to be that of Andrew Gilligan of the BBC. Mr Gilligan was very exercised this week by the differing bits of advice being given to various port authorities, and I felt that he was suggesting – in effect – that the Government ought to tell us everything that it knows and then we can make up our own minds how to respond.

It's a radical idea. Every bit of information that the Government gets, we are entitled to see. We should get the latest, and then the latest latest. In the last few months how many British people have travelled on American trains completely unaware that – at the very least – they ought to be looking out for al-Qa'ida operatives who seem to be Westerners? Think of the conversations struck up with potential terrorists through pure ignorance!

That must be wrong. If we judge that the FBI has a poor predictive record, we could decide not to worry about it at all. Or even – once a warning has been given about somewhere – that that is now the last place the terrorist will attack. We might rate Mossad intelligence sources above, say, the Estonian Special Branch's, while querying their motives. And how good do we think our man in Havana actually is?

David.Aaronovitch@btinternet.com