The leaked warning, contained in a bulletin issued by the US Department for Homeland Security last week, says the attacks aim to create catastrophic damage at about the time of the fourth anniversary of the September 11 attacks on New York and Washington.

The warning came as it emerged that the British Department for Transport had for the first time issued guidelines ordering a tightening of security around the UK road tanker fleet.

The US warning has been circulated among law enforcement agencies and fuel transport agencies. Although a preamble states that “no other intelligence exists to corroborate this specific threat”, the intelligence report is highly specific.

It says: “Al-Qaeda leaders plan to employ various types of fuel trucks as vehicle-borne improvised explosive devices (VBIED) in an effort to cause mass casualties in the US (and London), prior to September 19. Attacks are planned specifically for New York, Chicago, and Los Angeles. It is unclear whether the attacks will occur simultaneously or be spread over a period of time. The stated goal is the collapse of the US economy.”

The document goes on to suggest that the proposed methods will involve suicide drivers: “Some of the vehicles used will be hijacked. The type of vehicle may be anything from gasoline tanker trucks to trucks hauling oxygen and gas cylinders. Water trucks filled with gasoline or other highly combustible material may also be used. The detonation of the vehicles will be carried out by driving them into gas stations or ramming explosive-laden vehicles into the trucks carrying the fuel.”

The intelligence report says that the terrorist cells thought to be planning the attack will “execute the plan upon receipt of an order”. It goes on to speculate that the videotape released last week by Ayman al-Zawahiri, Al-Qaeda’s deputy leader, may have been meant as “the activation signal to the cells”. In the video al-Zawahiri warned that attacks would continue in Britain until it pulled out of Iraq.

The report says that Khalid Sheikh Mohammed, one of the alleged masterminds of the September 11 attacks, has told US interrogators that he had developed plans for targeting petrol stations. This was “due to their apparent vulnerability and the potential destructive force of a fuel-driven explosion”, it says.

The use of petrol tankers as mobile bombs has been a well-tested Al-Qaeda tactic in the Middle East. Terrorists in Tunisia, Saudi Arabia and Iraq have all used large fuel tankers against military and civilian targets.

A fuel tanker attack on the Khobar Towers in Saudi Arabia in 1996 killed 19 US servicemen. Four weeks ago terrorists exploded a fuel tanker in a busy market town 25 miles south of Baghdad killing nearly 100.

Although the specific threat of a tanker attack on London is thought to be new, Scotland Yard and MI5 have long feared that Al-Qaeda would try vehicle attacks on key targets in the capital.

Last year police disrupted an alleged plot to bomb a “soft target” — thought to be a Soho nightclub — with a truck bomb. More than half a ton of fertiliser, which can be used to make explosives, was recovered in a raid in north London.

Security sources say that fears about the use of fuel tankers has led to them being closely monitored when they enter the City of London.

Concrete security barriers have been placed in other key locations across the capital to stop vehicles packed with explosives reaching buildings such as parliament.

The Americans have previously been fearful that terrorists might use commercial vehicles for bomb attacks and warn that delivery vans could gain easy access to high-value economic targets. The FBI has also said that terrorists could use limousines packed with explosives to get near VIP targets.

The British Department for Transport issued new guidance on July 1 to prevent fuel tankers being hijacked and used as weapons. The security measures require carriers to be properly identified and transit sites to be made secure. All relevant staff are to be given security training. The measures apply to all dangerous goods transported by road or rail.

In California and Australia the authorities are introducing remote-controlled shut-down devices to stop any fuel tanker if it is hijacked. In Singapore the government has just begun putting tracking devices on petrol tankers to monitor their movements. Details of the latest intelligence warning were leaked to the American media last week, but no mention was made of the threat to London. The bulletin said the “stated goal is the collapse of the American economy”.

The disclosure of the warning has led to a disagreement among officials about the seriousness of the threat. Senior officials in Washington who were briefed on it last week said it was described as specific enough to warrant attention.

The FBI cautioned that the source of the information was not necessarily reliable. They said that the specific threat of a tanker attack to mark the anniversary of September 11 could not be verified.

This weekend British officials said they were unwilling to make any detailed comment on the warning. One government official said he knew of no specific intelligence warning of a fuel tanker attack in Britain: “It’s obviously a particular type of Al-Qaeda modus operandi used. But it hasn’t been used in Europe before.”

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