AL-QAEDA terrorists planned to hijack a passenger jet from eastern Europe and fly it into a packed terminal at Heathrow airport, killing hundreds of people, security sources have revealed.

The plot, which was taken so seriously that ministers considered shutting down the airport, helps to resolve the long-standing mystery of why Tony Blair ordered armoured vehicles and hundreds of troops to be sent to Heathrow in 2003.

It has now emerged that MI5 received detailed intelligence in February 2003 about a two-pronged plan to target Britain because of its decision to send troops in support of America’s invasion of Iraq.

The second element of the operation, inspired by Osama Bin Laden, involved a mortar attack on a departing passenger plane. It was organised by Khalid Sheikh Mohammed, the Al-Qaeda operations chief and mastermind of the September 11 attacks, the sources said.

Details of the plot have been provided by British sources after the White House issued a list of 10 Al-Qaeda plots foiled by America and its partners, including Britain.

Referring to what it called “the Heathrow airport plot”, the White House said that America and several partners had “disrupted a plot to attack Heathrow airport using hijacked commercial airliners”.

At the time cabinet ministers, including David Blunkett, then home secretary, said they could not divulge the nature of the threat but insisted that it was “real”. This weekend security sources gave fresh details of what they described as a double plot to kill hundreds of Heathrow passengers and punish Blair for Britain’s role in Iraq.

One said that an Al-Qaeda cell had been spotted carrying out reconnaissance at an airport in eastern Europe, possibly in Poland, Latvia or Estonia.

The source said: “The idea was to hijack the plane from somewhere that didn’t have the same level of security as airports in western Europe. The plan was to fly it into a terminal at Heathrow where there would be a lot of fatalities.”

The decision to send Scimitar armoured vehicles and 450 troops from the Household Cavalry and the Grenadier Guards was the first time since 1994 — when the IRA had tried to mortar the runway — that the army had been deployed on home security duties.

Blair was accused of staging a stunt to boost support for the invasion of Iraq. Although the troop deployment was largely symbolic, security would have been tightened up at East European airports, and, since 9/11, RAF fighter jets have been on standby to shoot down hijacked passenger planes.

Ministers could not disclose the nature of the threat for fear of compromising intelligence sources. The only explanation until now was that the troop deployment had been intended to thwart a possible mortar attack on a departing plane.

The new disclosures make it clear that this was just a part of a “twin track” Al-Qaeda plan to take the world’s busiest international airport out of operation.

Scotland Yard sources previously linked Andrew Rowe to that plot. He was a “dedicated jihadist” and Muslim convert who was jailed last month at the Old Bailey for terrorist offences. Rowe was arrested on a coach inside the Channel tunnel and was found to be carrying coded instructions on how to fire mortars. He was sentenced to 15 years after being convicted of two offences of having articles for use in terrorism.

Until his capture two years ago, Mohammed was a central figure in Al-Qaeda’s war on the West. He was inspired by the 1993 attempt to blow up the World Trade Center in New York. Mohammed conceived the September 11 plot, which he described as “far more successful than we had ever imagined”.

Last year The Sunday Times obtained transcripts of Mohammed’s interrogation in the months after his capture in Pakistan in 2003. They show that the original plan for September 11 envisaged the hijacking of 10 planes, which were to be crashed into targets on the east and west coasts of America.

In the interrogation, Mohammed also referred to an unspecified plot against Heathrow designed to punish Blair, whom he said Bin Laden called his “principal enemy”.

He told his interrogators that Al-Qaeda operatives had been given money and told to begin surveillance of Heathrow, assessing its weak points and finding locations from which planes could be shot down.

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