MI5's chief has made an unprecedented public statement defending the security service amid damaging revelations from a terrorism trial.
On Monday one of the longest ever UK trials ended with five men convicted of a terrorism plot to bomb Britain.
But details also emerged that revealed MI5 had follow two of the London suicide bombers during the probe.
MI5's head Jonathan Evans denied the service was complacent and published a rebuttal of criticisms on its website
In the rebuttal, MI5 rejects allegations that it failed to pick up on warning signs after Mohammad Sidique Khan and Shehzad Tanweer, two of the July 2005 suicide bombers, came onto their radar a year before.
MI5 officers followed the men and took their photographs and partially identified them, according to details that emerged during the massive year-long fertiliser bomb plot trial.
The surveillance occurred as spies watched the fertiliser bomb plotters with Khan secretly recorded talking to the chief conspirator, Omar Khyam.
In his statement on the MI5 website, Mr Evans said he welcomed the conviction of five men who had been plotting to build a massive homemade bomb. All of the men were involved in a broader network of so-called "jihadi" Islamist extremists and some of them had links to Al Qaeda and the Taleban in Pakistan and Afghanistan.
Mr Evans said the verdict was "recognition" of the hard work of his staff.
"But the severity of the threat facing our country means expanding counter-terrorist operations at an unprecedented rate just to keep pace. We calculate the number of those with similarly violent intentions to those convicted today has increased substantially since 2005.
"My service has never been complacent. The attack on 7 July in London was a terrible event. The sense of disappointment, felt across the service, at not being able to prevent the attack (despite our efforts to prevent all such atrocities) will always be with us."
Details to have emerged from the trial raise questions about how much MI5 knew about the two suicide bombers who came onto their radar, including their identity, associations with other jihadi extremists and their decision not to follow the men amid huge operational pressures in 2004.
Investigations by the BBC raise questions over whether MI5 warned West Yorkshire Police to investigate Khan after spies followed him home in early 2004 - but appeared to lack intelligence as to the threat he posed.
Critically, doubts have also been raised over a major Parliamentary report into the intelligence picture.
In 2006, Parliament's Intelligence and Security Committee gave MI5 a clean bill of health over the London bombings.
The report said that MI5 had told the committee that none of the of the 7 July bombers had been "named and listed" by the security service.
Home Secretary John Reid said that Mr Murphy confirmed that the findings still stood. But he added the committee would "reappraise all of these matters and questions."
But shadow home secretary David Davis urged Mr Reid to "think again" because the security services had initially briefed the press in the wake of 7 July that the bombers were all unknown to them.
"We now know that was not true," he said. "There will never be a 100% guarantee against terrorism. We don't expect it. But some mistakes are inevitable and some are not."