Risk of attack will remain

The state visit by President Bush has triggered the biggest anti-terrorist operation to defend a single human being ever seen in the capital. Preparations range from the shifting of concrete blocks to bombers, helicopter gunships and limousines with the armour of small battle tanks.

While the US secret services and intelligence agencies are focused on the possibility of an attack on the President, the biggest worry for their British colleagues is that the terrorist will use him as a diversion, and attack elsewhere.

British intelligence and antiterrorist experts now believe that an attack by al Qaeda is all but certain at some time in the near future. And the chances are they will attack a soft but symbolic target in London or major regional city.

The alert state has been raised to "severe", the second highest level - short only of an alert against an identified attack known to be on its way against a known target. Last month, Eliza Manningham-Buller, the head of the home intelligence service MI5, warned an attack by "Jihadist" supporters of Osama bin Laden was all but certain in Britain in the next year or so. She has suggested this might be with a crude biological or chemical device.

Her warning is one of the most specific uttered in public by an intelligence chief. She does not suggest the attack by al Qaeda in Britain would necessarily be related to Mr Bush's stop-off this week.

His presence in London would provide a powerful incentive for the extremists to stage the long-predicted "spectacular" in Britain, US or Europe. On the other hand, some intelligence experts believe security this week - FBI, CIA, British Intelligence, Anti-Terrorist branch of the Metropolitan Police and SAS - to be too tough for the terrorists. Al Qaeda and its friends have been increasingly bold in what is now a pattern of bombing against highvalue targets in the Middle East, from the Italian base at Nasiriyah in Iraq, the compound in Riyadh in Saudi Arabia, and Saturday's suicide bombing of the synagogues in Istanbul. Car bombs and suicide bombers travelling by motorbike and on foot must be the first concern.

The biggest worry is any attack on the visitors or their hosts would come from an unexpected direction. According to Dr Sally Leivesley, a former Australian government security analyst who runs her own consultancy, an attack might come from the air, from the Thames or even underground drains or power ducts. Most experts believe an attack now would be with high explosives and conventional weaponry.

Anti-terrorist units will be watching the soft targets as much as the hard ones - and this is likely to be a key feature of the strategy of the new Civil Contingencies Committee tasked with home defence. The list of such targets - almost anything to do with public utilities - runs into hundreds of thousands. With the new wave of car bombings, the pattern is clear. As Ms Manningham-Buller has underlined, the threat will still be there long after President Bush's visit ends.