Aleksander Litvinenko, who defected to Britain six years ago, is fighting for his life in a London hospital. A toxicology test at Guy’s hospital last Thursday confirmed the presence of the odourless, tasteless poison.

A medical report obtained by The Sunday Times shows that he has three times the maximum limit in his body, a potentially fatal dose. It is as yet unclear how the poison was administered, but on the day he became ill his family say he had a meal with a mysterious Italian contact.

Friends of Litvinenko, a former lieutenant-colonel in Russia’s Federal Security Service (FSB), are convinced that he is the victim of a murder attempt by former colleagues. They regard it as similar to the plot in which Georgi Markov, the Bulgarian dissident, was killed in 1978 with a poison-tipped umbrella on Waterloo Bridge in London.

Scotland Yard detectives have been liaising with consultants at Barnet hospital, north London, who have been treating Litvinenko since the poisoning on November 1, the anniversary of his defection.

A police spokesman confirmed an inquiry had been launched last week: “The Specialist Crime Directorate are investigating a suspicious poisoning.”

Supplies of thallium in Britain are highly restricted and cases of poisoning are extremely rare. One gram is enough to kill even the fittest of men and Litvinenko, 43, has all the symptoms of the poison, which can be diagnosed only after at least two weeks.

He has kidney damage, is constantly vomiting and has lost all his hair. He has also suffered severe damage to his bone marrow and an almost total loss of white blood cells which are vital to the immune system.

Doctors say these latter symptoms could suggest the presence of a second unknown agent in a potentially lethal “cocktail”.

In an interview last week at his bedside in the cancer ward of Barnet hospital, where he was being treated under a different name, Litvinenko said he believed it was a murder plot to avenge his defection.

“They probably thought I would be dead from heart failure by the third day,” he said. “I do feel very bad. I’ve never felt like this before — like my life is hanging on the ropes.”

Litvinenko claimed political asylum in 2000 and was granted British citizenship last month. One of the highest profile defectors from the FSB, he is on the wanted list in Moscow where he has made powerful enemies with his criticism of President Vladimir Putin.

Last month Litvinenko received an unexpected e-mail from a man he knew as Mario, an acquaintance he had made in Italy. The Italian said he wanted to meet him in London because he had some important information about the murder of Anna Politkovskaya, a Russian investigative journalist who was killed in the lift of her Moscow apartment block.

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