Brazil has demanded an explanation from Britain after police, searching for four men suspected of trying to bomb London's transport system last Thursday, shot Brazilian Jean Charles de Menezes, 27, at an Underground railway station in south London.
"I think we are quite comfortable that the policy is right, but, of course, these are fantastically difficult times," Metropolitan Police Commissioner Ian Blair told Sky Television.
Asked if the instructions were to shoot to kill if police believed a suspect was a bomber, he said: "Correct. They have to be that."
"It's still happening out there, there are still officers having to make those calls as we speak... Somebody else could be shot," Blair added.
Former British foreign secretary Robin Cook, who quit Tony Blair's government in protest over the Iraq war, told BBC television the blunder dealt "a very serious blow to our relations" with Brazil.
Thursday's failed bomb attacks, which killed no one but caused mass panic, came two weeks after bombers killed 52 people on London's Underground rail network and a bus.
Police had earned widespread praise for their handling of the inquiry, but the killing of an innocent man has raised concern about the trade-off between human rights and security.
Muslim leaders fear members of
their community will be targeted
Muslim leaders fear members of their community will be targeted after police identified the four men who blew themselves up on 7 July as British Muslims.
"To give licence to people to shoot to kill just like that, on the basis of suspicion, is very frightening," said Azzam Tamimi of the Muslim Association of Britain.
Former London police chief John Stevens wrote in the News of the World newspaper that he had sent teams to Israel and other countries hit by "suicide bombings" to train in combating them.
The London-based Islamic Human Rights Commission has expressed its concern that such training was taking place.
"When British police officers receive training from the forces of an apartheid state which uses assassination as official policy, killings such as this can only be expected as a natural consequence," said IHRC chairman Massoud Shadjareh.
Shadjareh added that "the killing may only be the first in a series of police killings in the post-7-7 era".
"They had to kill someone to show the whole population they are working and make the country safe"
The IHRC fears that innocent people may lose their lives because of the shoot-to-kill policy.
"They had to kill someone to show the whole population they are working and make the country safe," Alex Pereira, Menezes' cousin, told BBC Television.
"I ask the people to ask the Metropolitan Police and Tony Blair and everybody responsible for that: 'What kind of job are they doing?'"
The police chief Blair noted that the two sets of London bombings had followed a similar pattern but said no definite link had been established.