The police "shoot-to-kill" strategy will not be used indiscriminately, a key architect of the policy has said.
Jean Charles de Menezes was shot eight times
Tactics are 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 that shooting a suspect in the head was sometimes the "proportionate" response.
Ms Wilding spoke following criticism of the death of innocent Brazilian Jean Charles de Menezes at Stockwell Tube.
Relatives and friends of Mr de Menezes, 27, who was shot in the head seven times after officers wrongly suspected him of being a suicide bomber, have marched in his home town in Brazil demanding that arrests be made.
Tony Blair said he was "desperately sorry" for the death, but insisted that the police must be supported as they carry out anti-terror operations "in very difficult circumstances".
The shoot-to-kill policy, codenamed Operation Kratos, was put in place six months after 9/11.
Ms Wilding denied suggestions that it was an Israeli tactic and insisted that it was put together following extensive research in countries which had already suffered suicide bombings.
It also draws on extensive tests of different bullets and explosives, concluding 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.
A strategy for tackling suicide vehicle bombers is still being worked on and will eventually be adopted by every force in England and Wales, said BBC crime correspondent Neil Bennett.
UK police are usually instructed to shoot at the body, but that is not always appropriate when someone is carrying a bomb, Ms Wilder said.
Brazil has seen protests following Mr de Menezes' death
"If you go for the body mass, what is going to happen to those explosives? It's likely to go off," she told BBC News.
But she said the police response had to be proportionate, because "we always have to be able to answer, have we used reasonable force in the light of the intelligence of the situation and the risk?".
The dedicated senior officer, working with those at the scene, has to decide what the best course of action is according to the particular circumstances.
The policy is constantly reviewed in the light of information from other attacks, including the recent bombings in Egypt and those in Iraq, she said.
Debate over the police "shoot-to-kill" policy intensified following the death of Mr de Menezes, who ran from police when challenged.
Human rights lawyer Imran Khan called for a radical change in policy.
"There's been a mistake, mistakes have been made; my fear is the commissioner of the Metropolitan Police is saying that this could potentially happen again," he said.
"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.