Police have dealt with 250 suicide bomb scares since the 7 July London attacks, Metropolitan Police commissioner Sir Ian Blair has said.
Mr de Menezes was shot eight times after being mistaken for a bomber
Sir Ian indicated to Channel Four News that on seven occasions police had been on the brink of acting.
"We have got as close to calling it as 'that' and we haven't", he said.
He said the death of Brazilian Jean Charles de Menezes, wrongly thought to be a bomber, was a "dreadful mistake" but police had to protect the public.
"I know there have been 250 incidents since 7 July where we have considered whether we are seeing a suicide bomber," Sir Ian said.
"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.
"This is professional judgement. There are suicide bombers there and we have a job to do. It is a tragedy that should not divert us from the main issue," he said.
A shoot-to-kill policy for UK police, codenamed Operation Kratos, was put in place six months after 9/11.
Mr Menezes, 27, was shot in the head seven times by officers on Friday on a Tube in Stockwell, south London, when he was mistaken for a suicide bomber.
Relatives and friends have marched in his home town in Brazil demanding that arrests be made.
UK Prime Minister Tony Blair has said he is "desperately sorry" for the death, but insisted that the police must be supported as they carry out anti-terror operations "in very difficult circumstances".
Earlier, a key architect of the police "shoot-to-kill" strategy said it would not be used indiscriminately.
Tactics were constantly reviewed and any decision to allow police to kill must come from a senior officer, South Wales Chief Constable Barbara Wilding said.
But the threat of suicide attacks meant shooting a suspect in the head was sometimes the "proportionate" response, she said.
The strategy draws on tests of different bullets and explosives, which conclude that the most effective way of stopping a suspected suicide attacker from setting off a device is to fire at the head.
This is partly because acetone peroxide - the explosive used in the past by suicide bombers and linked to the attacks in London - is so volatile.
Debate over the policy intensified following the death of Mr Menezes, who ran from police when challenged.
Human rights lawyer Imran Khan called for a radical change in policy, after police refused to rule out such a mistake happening again.
"Now what does that do to the community, the community is not going to be confident in a police force that admits it can make mistakes."
Labour peer Lord Ahmed warned that illegal immigrants would try to run away if challenged by police.
"And whilst we need to catch those illegal immigrants or asylum seekers, nevertheless we can't shoot them because they're not terrorists," he said.