The legal advice is contained in confidential legal papers prepared for the Metropolitan police Special Branch and chief constables.

More than 150 pages of documents, seen by The Sunday Times, detail Operation Kratos People and Operation Clydesdale, the secret guidelines on dealing with suicide bombers. They provided the justification for the operation that led to the accidental shooting of Jean Charles de Menezes, an innocent Brazilian, at Stockwell Tube station in London last July.

This weekend Shami Chakrabarti, director of Liberty, the human rights group, said that she had resigned from the review panel set up by the Association of Chief Police Officers (Acpo), which last week declared the guidelines “fit for purpose”. She said: “It was window dressing, the review was a sham.”

The leaked documents reveal that at the time of the shooting, Scotland Yard’s firearms officers and senior commanders were acting on advice from one of the government’s top lawyers: that they could mount a successful defence to murder or manslaughter charges even if they killed a person who was not carrying a bomb.

The police had been told that they did not have to prove that they had acted reasonably in shooting dead an unarmed person. All they had to show was that they “believed” they were acting reasonably, a much more liberal level of defence.

Critics say that the advice amounts to a licence to kill innocent or unarmed people. In Israel, by contrast, police have to demonstrate that a suspect is actually carrying a bomb before they are permitted to open fire.

Chakrabarti said someone should be held accountable if police shoot and kill entirely innocent people: “No one is suggesting that an officer acting reasonably and in good faith should face murder charges. But if, as this leaked document suggests, the Acpo guidance is less stringent than in Israel — if it allows lethal force without firm belief that a suspect is carrying a bomb — those responsible for this system will have to carry the can.”

The controversial guidelines were drawn up by Acpo. In the wake of the September 11, 2001 attacks in America, it was searching for a legal framework to deal with the threat of suicide bombers posed by the rise of Al-Qaeda.

The advice by David Perry, a leading Treasury counsel, will reassure police marksmen and their commanders that even if they make critical “mistakes of fact”, they can be confident of having a good defence to possible murder charges.

Perry was asked by Special Branch to give the government’s legal opinion on a hypothetical scenario in which two suspected suicide bombers target a Westminster conference hosted by Tony Blair.

“If a police officer genuinely believes that a person is in possession of explosive substances and poses a danger to the lives and safety of others, the defence (of self-defence) would be available if, for example, the police officer shot that person,” Perry said.

“The defence would be available to the commander who gave the order to shoot and the officer who shot the suspect.”

Perry advises that the police do not have any duty of care to the public. They are thus not liable for damages for killing an unarmed person and failing to shoot the real bomber, as in the scenario.

That advice is likely to help both the two Scotland Yard marksmen who shot de Menezes in the mistaken belief that he was one of the failed Tube bombers of July 21, and Commander Cressida Dick, the senior officer who ordered them to stop him.

The Crown Prosecution Service is considering whether they and at least 10 other officers involved in the shooting operation should be charged with murder or manslaughter.

The Kratos guidelines also reveal the detailed advice on how to spot a suicide suspect.

Officers are told to look for people who may be sweating or look “recently clean shaven (with) short hair”. According to the document, suspicious behaviour includes “mumbling, possibly praying, looking anxious, wearing bulky clothing not in keeping with the weather, (and) holding something in the hand/clenched fist; wire or toggle protruding from bag.”

Police are told they must stand at least seven metres from a suicide bomber if they are to survive blast injuries. They are warned that even if a suicide bomber is shot dead, an accomplice may be waiting in the wings to detonate the bomb by remote control.

Firearms officers who may confront a suicide bomber are told: “It seems like an impossible task. But there are things you can do.” The recommended course of action is not to challenge the bomber but to “neutralise” him or her by firing a “critical head shot”. It adds: “Immediate incapacitation is essential as our objective is to prevent detonation.”

In the fictional scenario, Blair is hosting a conference on Afghanistan at the Queen Elizabeth II centre in Westminster. With him are Jack Straw, the foreign secretary, Donald Rumsfeld, the American defence secretary, and the French and German foreign ministers. They are all under threat from a cell of Al-Qaeda suicide bombers in a case study intended to provide practical guidance for British police chiefs on how to operate a “shoot-to-kill” policy.

The police sniper shoots dead a male suspect, who is subsequently found not to have been carrying a bomb. A second sniper then refuses to shoot a female suspect — thus failing to prevent her killing 15 people, including four police officers.

The scenario resembles a genuine Al-Qaeda plot to kill Tony and Cherie Blair, disclosed last year by Lord Stevens, former Metropolitan police commissioner. The aborted attack was scheduled for June 2002 while the Blairs attended the Queen’s Golden Jubilee celebrations outside Buckingham Palace.

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