Police fear that the strike, using a home-made chemical device, was imminent after an informant told MI5 that he had seen the lethal garment at the home of two young men.

Last night detectives were at the hospital bedside of a 23-year-old postal worker shot during a pre-dawn raid on his parents’ home, while his younger brother, aged 20, was being questioned at Paddington Green high security police station.

Armed officers who led the assault on the terraced house in Forest Gate, East London, wore oxygen masks and protective chemical gear after a tip-off from MI5 that the device had already been assembled.

Security chiefs are deeply concerned that there was no sign of the vest inside what they believe is a chemical bomb factory.

No weapons were found either as officers searched the two adjoining properties that have been converted to accommodate a large Bangladeshi family.

Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard’s Anti-Terrorist Branch, said that the raid, codenamed Operation Volga, was ordered in response to “specific intelligence”.

He said that there had been no time to conduct further surveillance, which suggests that the police believed a terrorist was close to launching an attack. The fear is that if chemicals were to be used then a likely target could be a train compartment on the London Underground.

Another theory is that a suicide attacker, wearing the vest under a shirt, could trigger the device in a crowded venue, such as a pub full of people watching an England World Cup match.

It is understood that the main target in yesterday’s raid was the postal worker, who was hit in the shoulder by a single shot fired by a police marksman.

His injuries are not life threatening and he remains under armed guard at the Royal London Hospital.

Scientific experts had to delay a detailed search of the house in Lansdown Road while officials from the Independent Police Complaints Commission moved into the property to begin their investigation into the shooting.

After the bungled inquiry into the shooting of the Brazilian electrician, Jean Charles de Menezes, who was mistaken for a suicide bomber a day after a failed terror attack last July, Scotland Yard chiefs are keen to ensure that procedures are followed.

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