Congestion charge and CCTV cameras around London will be used by police to track the journey of the car used in this morning's failed terror attack in the West End and to try to identify the driver, who ran off after abandoning the vehicle.

The job of anti-terrorist officers will be made easier by the number plate recognition system. This allows officers to input the registration of the Mercedes and find out where else it had been in the capital.

The CCTV cameras in Central London are linked to one of the most sophisticated systems in the world, which is based in a control centre off Leicester Square - a stone's throw from the scene of today's incident. The digital footage will show who left the car and when. It may also be clear enough to give police enough detail to issue a picture of the driver.

The car itself will be scoured for clues about the driver and for indications as to where it has been. Police are likely to issue photographs in a bid to get the public to help find the garage or the owner.

Car bombs have been a favourite weapon for terrorist groups from the Basque separatists ETA to the Provisional IRA, who launched their mainland campaign nearly 30 years ago with a coordinated series of attacks on the Old Bailey, Scotland Yard and three other sites.

But the IRA normally issued a warning. The big fear for Sir Ian Blair, the Commissioner of the Metropolitan Police, and his counter-terrorist experts, has always been that the Islamist cells would try to copy the savage tactics which have torn Baghdad apart and leave devices in densely-populated areas without warning.

There have been attacks in Europe using car bombs and they were also detonated in Istanbul several years ago.

At least two of the British gangs investigated and bugged by police and MI5 discussed mass casualty attacks which could be mounted with vehicles. The devices could be given extra potency by adding gas cylinders or chemicals to increase the effects.

A street like Haymarket, close to Piccadilly, would be an ideal target because it is constantly busy with traffic and passers by throughout the day and evening. The device would produce heavy casualties and spread fear.

If the plot is Islamist-inspired, Sir Ian and other police chiefs will be concerned at the possible consequences for inter-community relations: ever since the July 7 attacks officers have been concerned that a fresh device could produce a violent backlash.

But for the moment Cobra, the national emergency committee meeting this morning in Whitehall, will be assessing the first reports from the explosives experts who have examined the car and its cargo. They will be able to say whether the device matches what is known about Islamist bomb-making, and provide an estimate of its potental effects.

They will be wondering: if it was not Islamist, was it the work of an IRA splinter group still trying to carry on the 30 year war? Was it parked but intended to be taken somewhere else? Was there a timer? When was it due to explode?

In the meantime, while these questions are being answered, there are other immediate questions facing Cobra. So far the Yard has found no other suspect vehicles. So was the Mercedes a single attack, or part of a campaign ? Have the Islamist terrorists opened a new chapter of violence?

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