The former Met Police chief admits they appeared "way off the pace"
The police chief in charge of counter-terrorism at the time of the 7 July attacks has described the "guilt" he felt that they happened "on my watch".
The Met's former assistant commissioner Andy Hayman told the Times he felt "sickened" that the suicide bombers may have "slipped through our hands".
Anger turned to guilt when he realised they had been "on the radar", he said.
His comments follow a report by MPs which found MI5 did not have the staff to do extra checks on the ringleader.
The Intelligence and Security Committee (ISC) concluded there was insufficient manpower to check on the lead bomber, Mohammed Sidique Khan, before the attacks.
The bombings in London on 7 July 2005 killed 52 people and were the first suicide attacks on the streets of Britain.
Mr Hayman told the Times: "You can imagine the frustration and initial feeling of guilt and shock. Had they slipped through out hands?
"Four years on, I vividly recall the sickening feeling as I faced up to the possibility that we had missed opportunities to stop the 7 July bombers.
"We appeared to be way off the pace. So much so that, six weeks before the attacks, the national threat level was reduced from 'severe-general' to 'substantial'."
But he said he took reassurance from the ISC's comment that nothing more could have been done to avert the attacks.
"After a lot of soul-searching, I take greater strength in knowing that we did all that we could to track and thwart the terrorist threat," he said.
"Nevertheless, I can sympathise with the survivors and the families of the bereaved - several of whom I have met - and understand the pain and anger that they continue to feel."
Survivors and their families have rejected the findings of the ISC and are demanding a public inquiry.