Only a few hours earlier the Home Secretary had been haranguing critics of the Government’s anti-terrorism measures for failing to take the security threat seriously enough. “They just don’t get it,” he said.

He could have been forgiven for thinking as he picked up the telephone to inform the Prime Minister on holiday in the Caribbean: “They will get it now.”

So began a night of hightension drama, the culmination of an intelligence investigation that has been under way since early last year. The plot was unprecedented in scale but also horribly familiar, with all the hallmarks of al-Qaeda: sophisticated, ambitious, co-ordinated and entirely ruthless.

MI5 had built up a detailed picture of the plot over several months, but the threat had dramatically increased; the plotters had acquired the capability to carry out the attacks, perhaps within days. There was even a looming date, August 22, a holy day in the Muslim calendar and a propitious one for launching Armageddon.

From the Caribbean, Tony Blair spoke to President Bush. They had discussed the imminent plot at the weekend, but the Prime Minister had decided not to break off his holiday.

Just before midnight, as the first police raids began, Mr Reid chaired the first of two meetings of Cobra — Cabinet Office Briefing Room A, the emergency panel of officials from the police, intelligence and crisis management.

They were told an outline of the plot. Intelligence “chatter” had shown for some time that a huge terrorist attack was being planned, but the threat had come to a head.

The plotters allegedly planned to target at least five American aircraft heading for US cities by detonating bombs in mid-air; the suicide bombers allegedly planned to act alone, working from cells of four to five people. Instead of setting off the bombs simultaneously, Cobra was told, the terrorists planned to detonate them in a staggered pattern, to sow maximum panic while inflicting the greatest loss of life.

The final toll for the September 11 attacks in 2001 in the US was 2,749. Security sources told The Times that the suspected plot in Britain could have resulted in several thousand people being killed in the air and, possibly, on the ground.

The cells had been under surveillance since December, but before midnight on Wednesday intelligence was received that triggered a full-scale alert. The Cobra meeting was told that terrorists had developed the capacity to assemble the bombs.

A decision had to be taken urgently. “It was too risky to allow the surveillance operation to go on for any longer,” one source said. “The intelligence uncovered was really quite distressing.”

At 2am the security alert was raised to critical, the highest level, and airports were issued with the new security guidelines. By then police had begun to make arrests in London, Birmingham and High Wycombe.

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