Revealed: how London bombers slipped through MI5's grasp

Terror attack survivors and politicians demanded an inquiry into accusations that MI5 made mistakes that could have prevented the London bombings on 7 July in which 52 people died.

As mainland Britain's longest-running terror trial finally came to an end, the success of police and the security services in smashing an al-Qa'ida plot to bring carnage to Britain was overshadowed by the disclosure that MI5 also missed several opportunities to identify Mohammed Siddique Khan, the leader of the London suicide bombers.

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Five men from a home-grown al-Qa'ida cell were beginning life sentences last night over a plot to detontate home-made fertiliser bombs in a crowded nightclub and a shopping centre.

They also discussed trying to poison fans at a football ground, bombing gas supplies and obtaining an atomic weapon.

The five men found guilty of conspiracy to cause explosions were led by Omar Khyam, 25, from Crawley, west Sussex, described as the "head and heart" of the plot. His co-conspirators were Waheed Mahmood, 35, and Jawad Akbar, 23, both from Crawley; Anthony Garcia, 25, of Barkingside, east London; and Salahuddin Amin, 32, of Luton. Shujah Mahmood, 20, from Crawley, and Nabeel Hussain, 22, of Horley, Surrey, were cleared of all charges.

After the verdicts were announced, any jubilation by MI5 was tempered by the disclosure that its agents repeatedly encountered Khan while they investigated the fertiliser bomb plot. The link was not mentioned to the Old Bailey jury for fear of prejudicing their verdict.

In the wake of the 7 July bombings, the Government insisted there had been no warning of the attack and described the culprits as "clean skins" not known to MI5. But it emerged that the security services secretly bugged and photographed Khan in February and March 2004 but dismissed him as a peripheral figure. They also encountered a second of the 7 July bombers, Shahzad Tanweer, at least three times.

At one point Khyam and Khan were even recorded talking about terrorism. In the bugged conversation, Khan asked Khyam: "Are you really a terrorist?", to which he replied: "They are working with us."

BBC1's Panorama reported last night that MI5 agents even followed Khan to his home in Dewsbury, West Yorkshire, and took details of his car registration. But they failed to pass on the information to West Yorkshire Police.

MI5 last night released a detailed account of their operation against the bomb plotters. It insisted: "Even with the benefit of hindsight it would have been impossible from the available intelligence to conclude that either Khan or Tanweer post a terrorist threat to the British public."

The Prime Minister's official spokesman said Tony Blair had asked the Parliamentary Intelligence and Security Committee to re-examine the case. But John Reid, the Home Secretary, ruled out a public inquiry into the case, warning it would divert vital resources from efforts to thwart future terror plots. He told the BBC: "The implication that their identities were known at the time as people who were involved in any way in plotting a terrorist attack in Britain isn't true."

But Sir Menzies Campbell, the Liberal Democrat leader, said: "While the security service has a difficult role to play and cannot be expected to succeed every time, serious questions must be asked about key operational decisions."

David Davis, the shadow Home Secretary, said: "At the time of 7 July, we were told the suicide bombers were unknown to the authorities. That is plainly not true. They were suspected of criminal activities in support of terrorism."

Rachel North, who survived the blast on the Piccadilly Line train, said: "Now that we have discovered these men were very much on the radar of the security services and could have been stopped, that is going to be very difficult to come to terms with."

Graham Foulkes, who lost his son David on 7 July, said: "The consequences of that level of incompetence were such that my son was killed. That is truly appalling."

Sentencing the plotters, Judge Sir Michael Astill said: "You have betrayed this country that has given you every opportunity. All of you may never be released"

Jurors in the year-long trial heard of plans by the plotters to target the Bluewater shopping centre in Kent, the Ministry of Sound nightclub in London and the gas network with a fertiliser bomb. Britain's top anti-terrorist policeman, Deputy Assistant Commissioner Peter Clarke, said: "This was not a group of youthful idealists. They were trained, dedicated, ruthless terrorists who were obviously planning to carry out an attack against the British public."