By the time doctors finally discovered what had poisoned Alexander Litvinenko, he had only three hours to live.

As he lay unconscious, his wife Marina holding his hand and his ten-year-old son, Anatoli, stroking his forehead, a laboratory test on a urine sample identified the lethal element polonium-210 as the silent killer ravaging his body.

Doctors decided not to tell his family for fear of alarming them that they too might be at risk. Experts knew that although their seven-day search was over, there was no cure for the former KGB colonel. One source told The Times last night: “Once the polonium-210 was inside him, he had no chance of survival.” His life was nearing its end, but the concern now was for how many others might suffer.

A telephone call was made to the Government’s Health Protection Agency just after 6pm on Thursday, asking it to send its experts to the hospital immediately.

Police, wearing all-in-one suits — the CBRN equipment to guard against chemical, biological and radioactive contaniments — were also summoned to the intensive care ward.

The immediate task was to ensure that the nurses and doctors who treated the 43-year-old dissident were free from any radioactive contamination.

Forensic science teams sealed off the Litvinenko home in Muswell Hill, North London.

His family were not aware of the scale of the operation as, understandably, they were consumed by grief. Alex Goldfarb, an old friend who had kept a vigil at the hospital, said: “Marina [his wife] had tried to remain so brave and so positive, but now she sat staring into space. His son was in shock.”

Mr Goldfarb put a comforting arm around the shoulder of Walter Litvinenko, who had watched his son’s life ebb painfully away. Outside the hospital Walter Litvinenko described how a “tiny nuclear bomb” had killed his son. His rage was barely concealed as he described President Putin’s Government as “ a murderous danger to the world”.

Mr Litvinenko helped to distribute a powerful statement that his son had dictated from his death bed on Tuesday. His son’s friends, neighbours and political allies also lined up to condemn the Kremlin and its acolytes.

Scotland Yard’s counter-terrorist force were being more cautious. Officially, this was still no murder inquiry. It was an “unexplained death”. No mention was made that the intelligence services were now involved in this investigation. A post-mortem examination had to be postponed while health experts assessed the risks for the pathologist and his staff.

Hospital staff removed Mr Litvinenko’s body to a secure area while experts met to decide how, when and where the post-mortem could take place.

John Reid, the Home Secretary, asked experts from the Health Protection Agency to organise a swift, televised press conference to reassure the public. Professor Pat Troop, chief executive, said that the only ways the polonium-210 could have got into Mr Litvinenko’s system was through ingesting it, inhaling or taking it through a wound.

She insisted that no traces of the poison left through his sweat or urine could contaminate others. Barely an hour later came word that police had found polonium-210 at the Itsu sushi bar in Piccadilly, where he had dined with a friend on November 1, and the Millennium Hotel where he later met two Russians. It was also reportedly discovered at his home.

The revelations about polonium-210 provoked a new rush of conspiracy theories. Security experts said this was no crude grudge killing but was the work of assassins with likely access to a nuclear installation, not just to a radioactive isotope that could be acquired from medical waste.

On the day he was poisoned, Mr Litvinenko was reportedly handed a dossier bearing the name of the man who wanted him dead. He was a former comrade from KGB days, now working for “Dignity and Honour”, allegedly freelance hitmen who work for Russia’s secret services. President Putin broke his silence yesterday to deny that the Kremlin had played any part in the death.