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 which have been deployed to deadly effect by insurgents in Iraq.

Bombers in Baghdad have blown themselves up in hijacked petrol tankers and, in at least three attacks this year, have incorporated chlorine gas canisters in lorry bombs - turning improvised explosive devices into crude chemical weapons.

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 today that there was no specific intelligence to suggest that any kind of lorry bomb attack was imminent.

Counter-terrorism Command instituted the checks as a precautionary measure to try and prevent any such attacks.

They are now a routine part of Operation Mermaid, a long-running police operation aimed at ensuring vehicle safety.

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

“In addition to checking roadworthiness 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.

“As part of our day-to-day business we routinely carry out a range of measures, including road-side vehicle checks, to disrupt, deter and prevent terrorism, and to help create a hostile and uncertain environment for terrorists to operate in.”

One 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 underground car parks beneath hotels or office blocks.

Barot, 36, a convert to Islam, had submitted his schemes to the al-Qaeda leadership in Pakistan for approval but was arrested in 2004 before he could put them into action. He is now serving a life sentence.

To date, Islamist terrorist activity in Britain has been planned in 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.

“What we see around the world is that there is standardisation of terrorist tactics,” said Sandra Bell, director of homeland security at Royal United Services Institute.

“If something is seen as having been worked effectively then there is a support network, a training network and a resource network which is able to spread that know-how around the world very quickly. Terrorism is becoming more professional.”

In October 2005 the Sheraton and Palestine hotels in Baghdad were attacked and 20 people killed by suicide bombers driving a cement mixer.

More than 60 people died in July 2005 when a bomber blew himself up in a petrol tanker outside a Shia mosque in Musayyib, south of Baghdad.

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