Detectives are trying to piece together his movements using all available CCTV cameras along the route to see if anyone was shadowing him.

Alex Goldfarb, who spent days at the former KGB spy’s bedside, said: “This was not a one-man band. This was an incredibly sophisticated operation involving a lot of people.” Police admit they have not yet searched all the places Mr Litvinenko visited on November 1. Yesterday they examined the Sheraton Park Lane Hotel and another office address in Grosvenor Street — No 58.

Toxicologists have already detected the radioactive isotope polonium-210 at No 25, the headquarters of a private security company.

Investigators hope that this painstaking reconstruction will pinpoint where and when in the day he was poisoned.

Police are studying who was dining at the same time as him in the Itsu sushi restaurant in Piccadilly and who was around during his meeting with two Russians in the Pine bar at the Millenium Hotel in Grosvenor Square. A senior official with the Health Protection Agency said that it was hoped that computer models would be able to measure the strengths and amounts of polonium-210 found at places Mr Litvinenko visited to deduce when the poison was put into his system.

The source said “It takes a small number of hours for the person ingesting polonium-210 to begin to excrete radioactive material through sweat, saliva, urine or faeces.”

Mr Litvinenko was reportedly showing no signs of contamination when he left his town house in Muswell Hill, North London, that morning.

It was when he returned home in the evening that he complained of feeling unwell and went to Barnet Hospital where minuscule amounts of the polonium-210 were found.

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