This is what has been expected and feared for some months - that terror tactics honed on the streets of Baghdad would be visited on London and other Western targets.

The police and security services have been preparing for a vehicle-borne attack using either a car or, in the worst case scenario, a hijacked petrol or chemical tanker.

Earlier this year Sir Ian Blair, the Metropolitan Police Commissioner warned that “vehicle borne weaponry is the greatest danger that we can face”.

Counter-terrorism Command confirmed recently that it has been conducting security spot checks on tanker vehicles entering London for more than a year.

‘Items sent as earthquake aid’

Hidden among clothing were goods selected and purchased to support the activities of terrorists, a court was told

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BACKGROUND

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• Men in bungled raid say police attacked again

• More than Vigilance

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British security agencies are well aware that many young British Muslims have travelled to Iraq and Afghanistan to join the mujahidin and some of those who survive will return here with terrorist expertise.

There was no specific intelligence that a car or lorry bomb attack was imminent - hence the UK threat level remained at “severe” rather than “critical” - but the expectation has been that al-Qaeda cells in Britain would attempt to explode such a device.

The incident, which despite some early reports does not appear to have been a suicide bomb attempt, copies tactics used by two previous terrorist gangs.

Omar Khyam, jailed for life in April, discussed attacking the Ministry of Sound nightclub while Dhiren Barot, imprisoned last year, wanted to use stretch limousines packed with gas cylinders and explosives to blow up London landmarks.

The automatic assumption in the wake of this failed attack was that there are other devices yet to be discovered. The enduring hallmark of al-Qaeda is that it attacks multiple targets without warning with the aim of maximising casualties and publicity. London faces a day of disruption while suspicious vehicles are cordoned off and examined.

Compared to the days of the IRA’s British bombing campaigns, the car bomb at Haymarket appears amateurish.

The Mercedes cars contained several propane gas cylinders, large containers of petrol, a huge number of nails and some means of detonation to turn this cocktail of ingredients into a huge fireball.

But al-Qaeda is trying to operate in a climate in Britain which is more hostile to terrorist activity than ever before. High-strength hydrogen peroxide, which was used to make the 7/7 suicide bombs, is much more difficult to purchase than it was before July 2005.

The terrorists, however, are adept at finding ways - sometimes crude, sometimes quite ingenious - of turning everyday materials, cheaply and easily obtained, into bombs. The devices may be amateurish but security experts have no doubt that they are still lethal.

And the lessons these ruthless, ideologically-driven young men are learning in Iraq can only serve to make their future efforts more professional.

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