London - London's police chief has confirmed the police have adopted a shoot-to-kill policy to stop suspected suicide bombers detonating their explosives.

Police have admitted they shot dead an innocent man at an Underground station on Friday.

"This is not a Metropolitan policy, this is a national policy and 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 News television on Sunday.

"There are still officers having to make those calls as we speak," he said. "Somebody else could be shot."

"The only way to deal with this is to shoot to the head," Blair said. "There's no point in shooting at somebody's chest because that's where the bomb is likely to be".

He added: "There's no point in shooting anywhere else because if they fall down they detonate it. It is drawn on the experience from other countries including Sri Lanka."

Blair expressed "deep regrets" over the killing of a 27-year-old Brazilian electrician, Jean Charles de Menezes, who was shot dead in error by police who suspected he was a suicide bomber.

"This is a tragedy. The Metropolitan Police accepts full responsibility for this. To the family I can only express my deep regrets," Blair said.

Menezes was shot dead by police officers who had chased him into Stockwell Undergound station in south London on Friday after a surveillance operation.

Police are still holding two suspects in connection with Thursday's attempted attacks, but are continuing to hunt the four men believed to have carried the explosives, Blair said.

"They are still with us and will be for some time I think, but we are still anxious for any sighting of the four individuals or any knowledge of them or where they have been," Metropolitan Police Commissioner Ian Blair .

"There are inquiries all over London at the moment and indeed the rest of Britain."

Asked if the suspects were still in Britain, he said: "We have no reason to believe they are not."

He also said it had not yet been established that last Thursday's attacks were linked to similar bombings 2 weeks before that killed 52 people in London.

"We have no proof that they are linked but clearly there is a pattern here, isn't there, of four attacks - three on tubes and one on buses," he said. - Reuters and AFP