The shooting of a man at Stockwell by armed officers marks a watershed in the security of London.
By Jon Silverman
Legal affairs analyst
A man was shot dead at Stockwell station
A police confrontation with a suspected suicide attacker has been anticipated since 11 September and discussions have taken place at senior levels of Scotland Yard to determine the appropriate tactics.
Concern has also been expressed by the present commissioner and his immediate predecessor that the legal position regarding use of police firearms needs to be urgently reviewed in the light of the suicide threat.
The police deployment of firearms is governed by a manual published by the Association of Police Officers, last revised in February 2005.
It is not true to say that police officers must identify themselves or shout a warning when confronting a suspect believed to pose a grave and imminent threat.
The manual says that that procedure "should be considered" but recognises that the key aim of an operation is to "identify, locate, contain and neutralise" the threat posed.
In many situations, this would require the suspect to stop moving and put his hands in view.
'Central body mass'
That would not necessarily apply when the police are faced with a suspected suicide bomber.
The aim of opening fire is to stop an imminent threat to life. The most effective means of incapacitating a suspect is to shoot at the central body mass which contains the central nervous system.
This is what the police mean by "shooting to stop".
A head shot against a possibly moving target is more difficult to achieve.
However, the police have taken advice from officers in countries such as Israel and Sri Lanka which have long experience of suicide attacks.
Their advice is that if a suspect clearly has no intention of surrendering, the armed officer should attempt to aim for the head or lower limbs to prevent a suicide belt being detonated.
A former Home Office adviser, Dr Sally Lievesley, points out that suicide bombers represent the nightmare scenario for the police.
"A suicide bomber can kill four or five times more people than a conventional bomb because they can get so close to their targets, so different precautions and tactics are needed," she said.
The appropriateness of the tactics deployed at Stockwell and the intelligence on
which they were based will now be considered by the Independent Police Complaints Commission.
Its inquiry may well redefine the ground rules for the use of lethal force to protect Britain against the threat from suicide terrorism.