It amounted to what was then the biggest counter-terrorism operation in British history.

Thousands of hours of taped conversations, tens of thousands of man hours spent spying on members of the al Qaeda cell and scores of bugged telephone lines provided the evidence which eventually led to the five convictions yesterday.

Undercover surveillance took place on a scale not seen before, with officers on the trail of Omar Khyam bugging places where he stayed and his car. Police forces in southern England were involved in 28 raids on addresses on the day the suspects were arrested and in follow-up searches.

An intercepted exchange of web chat between Omar Khyam and Salahuddin Amin regarding the fertiliser confirmed that there was an active terror plot.

"It was decided that, instead of dispersing the cell, we would go after them and build a case to bring to court," said a senior police officer.

Listening devices for eavesdropping were placed in cars and houses, while CCTV footage was taken of the whole cell's movements. Some 3500 hours of audio tape were recorded, broken down into 60 transcripts, and then pored over by police officers.

It was a joint operation by the security services, Special Branch and the Metropolitan Police's anti-terrorism squad. A staggering 34,000 man hours of surveillance took place, with 7600 people involved in the investigation.

The court heard evidence from two members of the British security services, whose identities were shielded by a large screen.

As Operation Crevice wound up in the spring of 2004, MI5 officers began sorting through the mass of evidence they had assembled to check for leads to other plots and terrorist activity.

What the officers involved could not know is that it would lead to claims they missed the chance to stop the bombers who brought death and devastation to London on July 7 2006; claims they believe are inaccurate and unfair.

From the covert surveillance material gathered during the operation, the MI5 team picked out 55 individuals - mostly unidentified Pakistani men - whom they considered to be of further interest.

Of those, 15 were graded as "essential" because they had been overheard discussing terrorist activity with Khyam or his associates and so were subjected to further surveillance. Among the remaining 40 were two men, then unidentified, whose names were to become notorious: Mohammed Sidique Khan, ringleader of the July 7 bombers, and his right-hand man Shezhad Tanweer.

Khan was recorded meeting Khyam four times in the final stages of Operation Crevice. On one occasion, he even asked Khyam: "Are you a terrorist?" Tanweer also met Khyam around this time.

Security sources are adamant all 55 leads identified by the Crevice team were followed up, although they would not discuss how this was done in the cases of Khan and Tanweer. But they confirmed that routine practice would be to involve local police, in this case the West Yorkshire force.

What is clear is that, prior to the July 7 attacks, neither Khan nor Tanweer were identified as the men who spoke to Khyam. Security sources strongly deny that this represents an intelligence failure.

They insist that, given the information they had at the time, they could not have justified diverting more resources to investigating them. "There is a threshold for investigation and Khan and Tanweer only spoke to Omar Khyam about fraud and petty financial crime," one source said.

In July 2004 the pressure on those resources became even more acute as MI5 launched a new operation which led to the arrest and conviction of Dhiren Barot, who was sentenced to 40 years for plotting to kill thousands in terror attacks.

As the follow-up into Crevice continued, a montage of surveillance photographs taken during the operation - including one of Khan - was circulated by MI5 to other intelligence and security agencies.

One crucial individual who was not shown Khan's picture was Mohammed Babar, the supergrass' in the fertiliser bomb trial who was being held in the US.

Why his FBI handlers did not do so is unclear. Following the July 7 attacks, Babar did recognise Khan from his passport photograph as a man he had known from their time together in 2003 in a terrorist training camp in Pakistan.

Security sources say that even if he had recognised the picture, he knew Khan only as Ibrahim' and had no idea of his real identity.

But the sources do acknowledge they expected to be criticised in the aftermath of the Crevice trial, even though they regarded the accusations being levelled at them as unjust.