MI5 today expressed "profound regret" for failing to prevent the 7/7 bombings.
One of the Security Service’s most senior officers told the inquest into the attacks that every member of the agency lamented the fact that plot ringleader Mohammed Sidique Khan was not fully investigated before the atrocities.
The top spy, who can be named only as Witness G, insisted MI5 had "no inkling" of what was to befall London on July 7 2005 and said it would be "nonsensical and offensive" to suggest otherwise.
Hugo Keith QC, counsel to the inquest, said: "You have nevertheless recorded that it is a matter of profound regret that despite its efforts and industry, the Security Service did not manage to ascertain the full significance, and intentions importantly, of Mohammed Sidique Khan and thus did not manage to prevent the atrocities of July 7."
Witness G said: "Every member of the service feels that."
Khan and his number two, Shehzad Tanweer, were seen meeting known terror suspects 17 months before the July 7 2005 London bombings, which killed 52 innocent people.
Witness G was called to give evidence to the inquest about whether Britain’s security agencies could have drawn together intelligence about the pair’s links with extremists and established they were planning mass murder.
The top spy, who is chief of staff to MI5 director general Jonathan Evans, spoke of the large number of serious terrorist conspiracies under investigation around the time of the 7/7 attacks.
He described a plot to bring down transatlantic airliners foiled in 2006 as the gravest threat faced by the Security Service since the Second World War.
"In terms of the seriousness of the plot, I believe it is the most significant thing the service has been involved in since 1945," he said.