There have been 250 suicide bomb scares since the July 7 atrocities, according to the Metropolitan Police Commissioner.
Sir Ian Blair said there had been 250 incidents since the attacks when police thought they may have been dealing with a suicide bomber.
And he indicated that on seven occasions police had been on the brink of acting.
He said: "I know there have been 250 incidents since July 7 where we have considered whether we are seeing a suicide bomber".
"I know that when I last saw it there had been seven times when we have got as close to calling it as 'that' and we haven't."
Sir Ian said last week's killing of Jean Charles de Menezes had been a "dreadful mistake".
But he said there were suicide bombers out there and the police had to protect the public.
Earlier, armed police stormed a flat in Enfield, north London - 58 Curtis House - where two of the would-be-bombers from the 21 July attempts may have returned to their flat after the failed attack.
Forensic officers are investigating whether they picked up further explosives and supplies
A woman has claimed she saw three men outside the flat on the ninth floor of the tower block on Friday, July 22, looking "worried" and "suspicious".
That has led to the fear that at least one of the bombers may have returned to the property to retrieve explosive materials.
The flat is understood to have been let by 24-year-old Yasin Hassan Omar.
The suspected Warren Street bomber had reportedly been living there on benefits since 1999 and was frequently visited by Ibrahim.
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