One of MI5’s most senior officers expressed the “profound regret” of the security service at being unable to prevent the 2005 London bombings but rejected suggestions that there had been significant intelligence failings.

The senior spy, dubbed witness G, told the inquest into the 7/7 bombings, which killed 52 people, that the number of groupings or networks actively involved in plotting terrorist acts had grown dramatically between 2001 and 2005.

He said that in 2001 the security services had known of 250 primary investigative targets. This had jumped to 500 by July 2004 and to 800 by July 2005.

By contrast, when asked about the number of desk officers in the international counterterrorism section looking at Islamic work, he said it had numbered “between 10 and 99” in 2004/2005. The agency subsequently received an increase in resources as part of the government spending review in 2004.

Witness G accepted there “were aspects of the intelligence picture . . . that did not work as well before 2005 as it has done since”.

However, he rejected claims that there had been major intelligence failings.

Hugo Keith QC, counsel for the inquest, asked him: “Although you would not use the phrase, I think, an intelligence failing, it does indicate some level does it not, of a failing?”

Witness G replied to the court: “You can always get things better and I believe we have [now], but I wouldn’t accept that was a failure.”

The inquest has renewed public scrutiny of MI5’s role amid claims that the security services could have prevented the attacks.

The agency had been aware of Mohammed Siddique Khan, the July 7 2005 ringleader, and another of the bombers for over a year before the attacks but decided not to pursue them.

MI5 was exonerated by MPs in 2009 for deciding not to pursue Khan, but the report by the Parliamentary Intelligence and Security Committee also showed agents were alarmingly overstretched before July 7.

The inquest also heard that photographs of 7/7 bomber Shehzad Tanweer were shown to Mohammed Junaid Babar, who was in US custody but failed to identify Tanweer as an extremist who had attended a terrorist training camp.

Witness G also told the court that although there were in hindsight “seven or eight” references to a “Sidique Khan” picked up by various intelligence material, there were variants in the way the name was spelt. In addition there was no central system for pulling all these references together as various computer databases were not linked up.

He also told the inquest there were “plenty of Sidique Khans on our systems rather than one specifically in Batley” – the West Yorkshire town in which the bomber lived.

MI5 staff numbers and budgets have been boosted significantly since the attacks, but Jonathan Evans, the agency’s chief, has admitted his agents can still “only hit the crocodiles nearest the boat”. The MPs said MI5 would need “hundreds of thousands of agents” to pursue every lead.

The inquest continues and is due to end next month.

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