Though the days when all British bobbies were thought to be unarmed have long passed, the clinical and close-quarter nature of the shooting was unprecedented in Britain. Police have previously shot men believed to be dangerous, but they have not stood over a prostrate figure and unloaded five rounds into him from point-blank range. To compound matters the police admitted yesterday he had nothing to do with the terror attacks.

However, London has never before faced suicide bombers. The stakes have become much higher, forcing new rules of engagement.

Friday’s killing was a direct result of aggressive new guidelines from Scotland Yard based on the experience of Israel and Sri Lanka in dealing with suicide bombers. British officers are now under instructions to shoot suspects in the head if they are believed to be suicide bombers posing an imminent danger.

A policy of “shoot-to-kill” echoes the darkest days of the Northern Irish troubles. And it raises worrying questions when applied in the much larger and more mixed communities of mainland Britain, and when the suspected terrorists are much more elusive and shadowy.

Today’s Muslim leaders, although supportive of law and order, are worried and demanding explanations. “There may well be reasons why the police felt it necessary to unload five shots into the man and shoot him dead, but they need to make those reasons clear,” said Inayat Bunglawala of the Muslim Council of Britain.

Prophetically, a former senior Special Branch officer from Northern Ireland said: “I suspect that the authorities in England will make all the same mistakes as we did.”

Those errors include an operation in Gibraltar in 1988 when the SAS killed three IRA members in the belief that they were about to detonate a radio-controlled bomb. In reality the explosives were miles away and the three suspects were carrying no radio equipment.

Although the Gibraltar coroner’s court ruled that the killings had been lawful, the European Court of Human Rights later criticised the “lack of degree of caution in the use of firearms” by the SAS.

Specialist security forces, such as the recently formed Special Reconnaissance Regiment (SRR), which has been drafted in to combat the present terror threat, are generally protected by law if they shoot first and ask questions later, provided they believe the suspect was a threat to the lives of others.

This proved to be the case when Diarmuid O’Neill, an unarmed IRA man, was shot dead in his Hammersmith flat in 1996. The officer who pulled the trigger told a coroner’s court: “His body language was aggressive, he leaned towards me.” The jury returned a verdict of lawful killing.

But the new policy to cope with suicide attacks is a step further. With suicide bombers there is no question of trying to stop suspects by wounding them: only immediate execution will do.

The threat and risks run far wider than London. Specialist firearms officers are being deployed on secondment to MI5, which is opening eight offices in cities including Glasgow, Manchester, Liverpool, Leeds and Birmingham. The aim is to increase the surveillance of terrorist suspects and to penetrate radical networks with informants.

The highly secretive SRR draws on members of the 14th Intelligence Company, and the Force Research Unit (FRU), which handled all military intelligence informers in Northern Ireland.

If the pattern of Northern Ireland is repeated, Asian servicemen will be encouraged to volunteer for covert duties. Some may “resign” from the army to return to their communities as undercover agents.

The past two weeks have given Britons a test of what is potentially in store in the weeks, months, even years ahead. It is a dangerous balance for everyone.

“You can’t be afraid to act if life is at stake,” said a former Northern Ireland Special Branch officer. “But if you alienate people you can hand the terrorists a long-term support base from which to operate.”

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