The full extent of MI5's knowledge of the July 7 suicide bombers - and its failure to pursue them as priority targets - can be revealed for the first time today.
The truth about the failure of Britain's intelligence agencies to investigate the 7/7 terror cell has had to be kept secret until now because of reporting restrictions surrounding the fertiliser bomb plot trial.
It can now be revealed that Mohammed Sidique Khan, the ringleader of the July 7 bombers, was a close associate of the leader of the fertiliser cell, Omar Khyam, at a time when he was one of Britain's top terror targets.
The two met each other at least four times in England while Khyam was under surveillance by MI5 in the final stages of his plotting.
At one point they were even recorded by Security Service agents talking about terrorism.
Khyam also met another of the 7/7 suicide gang - Khan's right-hand man, Shehzad Tanweer - while under surveillance by MI5.
Yet despite this, neither Khan or Tanweer were classified as priority targets by the Security Service.
Graham Foulkes, who lost his 22-year-old son David on July 7, said that when he learned of the truth about 7/7 he was "absolutely overwhelmed with a sense of sheer disbelief".
He said: "The consequences of that level of incompetence were such that my son was killed. That is truly appalling.
"Could the bombings have been prevented? As a father who lost a son, I am drawn to that conclusion."
Once the investigation, codenamed Operation Crevice, had wound up, it was MI5's responsibility to assess the threat posed by those on the fringes of the operation and decide what level of resources to devote to investigating them.
But the service decided Khan and Tanweer were "peripheral" figures. They were listed among the dozens of "desirable" rather than core "essential" targets and, as a result, were able to slip the net.
Within 16 months they went on to kill 52 innocent people in the first suicide attack on European soil.
Khan and Khyam also attended a terror training camp together in Pakistan - a full two years before the 7/7 bombings.
It can also be revealed today that crucial opportunities to identify Khan as a serious terror threat in the months before 7/7 were missed.
A photograph of him was circulated by MI5 to intelligence agencies around the world in early 2005 - yet, astonishingly, it was not shown to the one witness who could have positively identified him.
American Mohammed Babar, the "supergrass" in the fertiliser trial, had been to the same training camp as Khan and Khyam. Following his arrest, he told the authorities that Khan, whom he knew as Ibrahim, was "trouble" and "should be checked out".
For an unknown reason, he was never shown Khan's picture by the FBI.
Babar was later said to have been angry about this failure when he heard that Khan led the 7/7 attacks on London.
In its official report on the 7/7 bombings, the parliamentary Intelligence and Security Committee (ISC) described this as a "missed opportunity" to identify the man who would go on to lead the biggest mass casualty terrorist attack Britain has ever seen.
In the wake of the bombings, the Government insisted there had been no warning of the attack.
The bombers were said to have been "clean skins" who had never made any significant impression on the intelligence radar before.
Today's revelations will throw into doubt MI5's ability to protect Britain from terrorism and heap pressure on its head, Jonathan Evans, who took over from Dame Eliza Manningham-Buller just days ago.
He served as her deputy during part of the period in question.
Dame Eliza announced last year that she would be leaving, with some commentators alleging she was quitting "just as the truth about 7/7 was about to come out".
Her departure is now likely to be seen as an attempt to allow the service to start "with a clean slate".
Furious survivors and grieving relatives renewed their calls for a public inquiry into the 7/7 bombings in the wake of today's revelations.
Rachel North, who survived the blast on the Piccadilly Line train, said she was "very shocked" and "appalled" when she first learned the truth about Khan and Khyam a few months ago.
She said it now appeared as though 7/7 could have been prevented.
"This has fuelled my desire for an independent inquiry (into the bombings) because it appears we have not been told the truth about what happened and what we knew about these bombers prior to 7/7," she said.
Mr Foulkes, whose son, a media sales manager for The Guardian in Manchester, was killed in the Edgware Road blast, added of Dame Eliza's departure from the top job at MI5: "She has fallen on her sword. I think she jumped before an inquiry was held, which undoubtedly would have criticised her personally."
The Tories also seized on the issue.
Shadow home secretary David Davis promised to table a series of parliamentary questions and called for an independent inquiry.
The public have been kept in the dark about the truth behind 7/7 for more than 18 months because of the fertiliser case. The fertiliser jury were allowed to know nothing of the links.
But it can be revealed today that: Khan and Khyam attended a terror training camp together in Malakand, Pakistan, in July 2003, where they undertook weapons training. The pair had also been planning to return to Pakistan in early 2004 when Khyam was arrested; Khan and Khyam met each other at least four times in the weeks before the fertiliser bomb plot was smashed. Although Khyam, the ringleader, was arrested, Khan was dismissed as a fundraiser and not pursued; At one point during their bugged conversations, Khan asked Khyam "Are you really a terrorist?" and he replied: "They are working with us." Tanweer also attended one of the meetings, along with Khyam's brother and co-accused; MI5 lost track of Khan between the bugged conversations and 7/7. He was allowed to slip off the radar - during which time he was cleared to work as a school mentor. There are also claims that West Yorkshire Police were asked to keep tabs on him by MI5 but failed to; Babar could have identified Khan as a terrorist months before 7/7 but was never shown a photograph of him by his FBI handlers.
Senior Government officials will now come under pressure to order an independent inquiry into the events of 7/7.
The police and MI5 are expected to issue a robust defence of their decision-making.
One police source said: "Yes, Sidique Khan and Tanweer were seen during the Crevice surveillance operation. From our perspective they were not priority targets because a) they were not part of the Crevice plot, and b) they did not pose a threat to the public.
"What was known about them was that they were involved in minor fraud, possibly for the purposes of fund-raising. The focus, and it had to be an unwavering focus, was on those who posed a threat to the British public.
"Obviously the question will be asked - if more investigations had been carried out into Sidique Khan and Tanweer, could the attacks on July 7 have been prevented? I would refer you to the ISC report which concludes...possibly.
"Given the intelligence available at the time, the decisions made were reasonable.
"We had to focus on those who posed a threat to the public. Decisions had to be made on a day-to-day basis about priorities - the priorities are determined by the level of threat posed by individuals and resources available.
"To keep one person under surveillance for 24 hours takes in the region of 50 people. We have to make difficult decisions."