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.

It is routine practice at the end of all such investigations to go through material to ensure that no potential new lines of inquiry are overlooked.

What the officers involved could not know is that it would lead to claims that they missed the chance to stop the bombers who brought death and devastation to London little over a year later - claims they believe are inaccurate and unfair.

At the time the MI5 team had every reason to feel pleased with themselves.

Crevice was then the biggest counter-terrorism operation to have been mounted by the Security Service in the United Kingdom.

It had culminated in the arrests of ringleader Omar Khyam and his fellow plotters and, they believed, averted an attack which could have cost numerous lives.

It was an operation which had imposed enormous strain on an organisation then numbering just over 2,000 staff.

"It sucked resources from across the Service, from the Met, from the regional police forces," one security source recalled.

From the huge volume of 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 were subjected to further electronic surveillance.

Among the remaining 40 were two men, as yet unidentified - whose names were to become notorious - Mohammed Sidique Khan, the 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 that 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.

They point out that, despite Khan's question, neither man had been overheard discussing actual terrorist activity in the UK with Khyam, who was also involved in various low-level criminal scams.

"There is a threshold (for investigation) and Khan and Tanweer only spoke to Omar Khyam about fraud and petty financial crime," one source said.

"When we had a sizeable number of individuals posing a direct threat to life in the United Kingdom, there was no way resources would be diverted from other cases to a man who was a criminal."

In July 2004 the pressure on those resources became even more acute as MI5 launched a new operation, which was to become even bigger than Crevice and 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 America.

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, also attended by Khyam.

But security sources say it was far from certain that he would have identified him from the poor quality surveillance photograph, even if he had seen it.

They also point out that even if he had recognised the picture, he knew Khan only as "Ibrahim" and had no idea of his real identity.

The sources acknowledged that they expected to be criticised in the aftermath of the Crevice trial - even though they regarded the accusations being levelled at them as unjust.

They strongly denied that the case had anything to do with the retirement of MI5 director-general Dame Eliza Manningham-Buller, who was said to have informed the then home secretary Charles Clarke in May 2005 of her decision to stand down this month.

They point to the choice of her deputy, Jonathan Evans, to succeed her, as a sign that ministers still have confidence in the way the Service operated and did not see the need to bring in an outsider to institute a fundamental shake-up.

Since 2004, the number of MI5 staff has risen to around 3,000, with a further 500 in the pipeline, while the service has also opened a series of eight regional offices around Britain.

At the same time, the terror threat has risen, with some 200 jihadist networks of various sorts now known by MI5 to be operating in the UK, as against just 30 in 2003 and 50 in 2004.

In such circumstances - even with the additional resources available - the sources warn that there can be no guarantee that another Mohammed Sidique Khan will not emerge undetected, with deadly results.