The controversial "shoot-to-kill" firearms policy is not being extended to non-terrorist incidents, Met police chief Sir Ian Blair has said.
But Britain's top officer said the police already had the power to shoot hostage-takers and kidnappers, for example, who were threatening others.
Sir Ian also said the statistics showed his officers were not "trigger happy".
Firearms officers used the strategy when they killed Brazilian Jean Charles de Menezes in error on the Tube.
They had mistaken him for a suspected suicide bomber at Stockwell station.
Answering questions from the Metropolitan Police Authority, Sir Ian said Operation Kratos - by which the policy of shooting suspected suicide bombers in the head is known - was not being extended.
But he said: "If somebody was holding a 10-year-old child with a knife to the child's neck and is about to start cutting the child's head off, the only shot available might be to the head, in which case that's what would be done."
This did not constitute a widening of the powers because the criminal law had always allowed reasonable force to be used and that had not changed.
He admitted that there needed to be a public debate on the lethal use of force and a leaflet to explain the policy has been sent out today to all areas of the Metropolitan Police.
And defending his force's record, Sir Ian said the statistics for the nine years since the firearms unit - known as SO19 - was set up, showed it had a good record.
He said SO19 officers fired their guns at one in every 2,000 incidents that they attended.
"We injure somebody once every five thousand occasions, and we kill somebody once in every ten thousand occasions.
"I really do believe that this level of press commentary that in some way the Metropolitan Police is trigger happy is not borne out."
The so-called "shoot-to-kill" policy sets out a range of options for officers when tackling a suspected suicide bomber.
A Scotland Yard review has concluded it remains the best approach.
Assistant Commissioner Steve House told the authority officers had no choice but to employ "shoot-to-kill" in terror-related situations.
He said: "I think we would be held in some contempt by the people of London if we were to turn around and say, I'm sorry we have no credible tactic to deal with this very real threat."
The Independent Police Complaints Commission is still investigating what went wrong when Mr Menezes was shot dead.
The shooting came the day after the failed London bomb attacks of 21 July.
The Brazilian electrician was shot seven times in the head by anti-terror officers while passengers looked on.
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