Security spot-checks are being carried out on petrol and chemical tankers, cement mixers and other vehicles that could be used by suicide bombers.

Police are monitoring lorries on key routes into London amid concerns that terrorists might copy tactics that have been used to deadly effect by insurgents in Iraq.

Bombers in Baghdad have blown themselves up in hi-jacked petrol tankers and, in at least three attacks this year, have used chlorine gas canisters in lorry bombs.

The checks follow a warning earlier this year by Sir Ian Blair, the Metropolitan Police Commissioner, that “vehicle-borne weaponry is the greatest danger that we can face”. But Scotland Yard stressed yesterday that it had no specific intelligence to suggest that a lorry-bomb attack was imminent.

The Metropolitan Police’s Counter Terrorism Command set up the checks as a precautionary measure but they have now been incorporated into Operation Mermaid, a long-running operation aimed at ensuring vehicle safety.

“A counter-terrorism element has been added to the routine work of checking vehicles carrying dangerous goods,” a police spokeswoman said.

“In addition to checking road-worthiness and the safety of the load, officers will also be looking to see if there is anything about a vehicle that makes them suspicious. There is constant monitoring of the methodology of terrorism overseas and we have seen vehicles like this used in attacks. It would be remiss of us not to be take that into account.”

An al-Qaeda terrorist convicted last year had been planning vehicle-borne bomb attacks in London when he was arrested. Dhiren Barot pleaded guilty to plotting a series of attacks, including detailed plans to pack stretch limousines with gas cylinders and explosives and detonate them in car parks beneath hotels or office blocks.

Barot, 36, is now serving a life sentence.

To date, Islamist terrorist activity in Britain has been planned and directed by al-Qaeda based in Pakistan. But security agencies are aware that a significant number of British radicals have travelled to Iraq to fight and some will have returned with expertise in conducting urban terror attacks.

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