The widow of the murdered former KGB spy, Alexander Litvinenko, has written to Vladimir Putin challenging the Russian President to prove he had nothing to do with her husband’s death.

In an emotional message Marina Litvinenko gave warning to the Russian leader, who has repeatedly denied that the Kremlin had any involvement in the murder, to stop sheltering the killers. She tells him: “I can assure you I will not rest until the murderers are brought to justice.”

Litvinenko was convinced that Mr Putin gave the orders for his murder, but his widow says that she is prepared to give the President the benefit of the doubt.

To prove his innocence she tells him to order his officials to cooperate with the British investigation. “If you and the Russian state are not responsible for the murder, surely you should be doing everything possible to assist the British authorities in discovering who is guilty?” she writes.

“I have never said that I knew you were personally responsible. I said that if you did not make every effort to assist the UK authorities in the discovery of the perpetrators of this terrible crime, I could only assume that you must have something to hide.”

Mrs Litvinenko, 44, has rarely broken her silence or been seen in public since the death of her husband from polonium210 poisoning in November. She and Anatoly, her 12-year-son, remain under police protection. Her intervention comes as Kremlin prosecutors have requested permission for a team of Russian detectives to visit London to carry out their own investigation into the former spy’s death. They want to question more than a hundred witnesses, including Mrs Litvinenko.

She has been offended by remarks made at the weekend by three Russian businessmen, alleged to be the prime suspects behind the poison plot.

Andrei Lugovoy, Dmitri Kovtun, and Vyacheslav Sokolenko, who all met Litvinenko at a London hotel the day he fell ill on November 1, told Ren TV, a Russian TV network, that if a sushi bar opens in Moscow it will have to serve “Litvinenko tea.”

The three men were also shown together last night on Russian national television’s NTV channel being interviewed in a Moscow sushi bar. Mr Lugovoy was also pictured firing a revolver at a shooting range used by employees of his security company.

All three men, who are said to have been contaminated by polonium210 — although Mr Kovtun claims to be free of contamination after treatment at a Moscow hospital — deny any involvement in the poison plot. They told Ren TV that they are heading to a ski resort to escape British media attention.

The trio also say that they are considering writing a film about the affair as Hollywood studios are already fighting over rival scripts.

They joked about who would play them in the film saying: “Lugovoy will be played by Brad Pitt, Hugh Grant will take the part of Sokolenko, and Kovtun will be played by Quentin Tarantino.”

A close friend of Litvinenko told The Times yesterday that he had received death threats. Yuri Felshtinsky, who wrote Blowing up Russia with Litvinenko, says that he has received telephone threats from a former FSB agent who said that “an accident could happen to you” at his home in Boston, in the US, where he has asylum.

“You have to take these people seriously. I told Sasha [Litvinenko] before he died that if we had written a book about poisoning dissidents with radioactive poison, people would have called us fantasists.

“Of course the Kremlin could have killed Sasha in a simpler way. But this is a project for them. They enjoy finding new ways to kill.”

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