A former Russian spy is fighting for his life in a London hospital while the Kremlin is dismissing as "sheer nonsense" allegations that Russia had a hand in poisoning him.

Former KGB agent Alexander Litvinenko was moved today to an intensive care unit in London as a precautionary measure after his condition deteriorated slightly overnight.

A photograph taken by Litvinenko's family today shows the former spy in his hospital bed, his head bald and plastic tubes plastered onto his chest.

An outspoken critic of the Kremlin, Litvinenko was apparently given thallium -- a toxic metal found in rat poison -- during a dinner meeting at a London sushi bar on Nov. 1.

Doctors at London's University College Hospital say he has only a 50 per cent chance of surviving.

Litvinenko, 43, had been investigating the killing of a Russian investigative journalist Anna Politkovskaya. He told reporters last week he fell ill on Nov. 1 after dinner with a contact who claimed to have details about the murder.

Friends of Litvinenko have alleged he was poisoned because he was critical of the Russian government.

But In Moscow, Kremlin spokesperson Dmitry Peskov told AP that the suggestion is "nothing but sheer nonsense."

He added that he would not comment on the poisoning itself.

Litvinenko also told reporters that Russia's Federal Security Service still operates a secret Soviet-era poisons lab in Moscow.

Alexander Goldfarb helped Litvinenko flee to Britain in 2000. In an interview with AP, Goldfarb said the former spy told him more details on Monday morning about the day he was poisoned during a telephone conversation from his hospital bed.

According to Goldfarb, Litvinenko briefly met two men from Moscow -- one of whom was a former KGB officer who he knew -- for tea at a central London hotel before dinner.

"I called Alexander in hospital ... he told me it is true, on that day, before meeting the Italian, he met with two Russians," Goldfarb told AP, adding Litvinenko had not previously met the second man.

Earlier Monday, Goldfarb told British Broadcasting Corp. radio that Litvinenko was poisoned because of his opposition to the Russian regime.

"It's very difficult to imagine the president's ordered the killing, it's true, and nobody's saying that (President Vladimir) Putin personally ordered it -- though it's very likely," he told the BBC.

Police said a specialist crime unit began an investigation Friday into how Litvinenko may have been poisoned. No arrests had been made, said a Scotland Yard spokesperson, speaking on condition of anonymity.

Britain's Press Association identified the Italian contact Litvinenko met at the restaurant as Mario Scaramella, an academic who has helped investigate KGB activity in Italy during the Cold War. Scaramella could not immediately be reached for comment.

Litvinenko co-authored a book in 2002 titled "Blowing up Russia: Terror from Within." In it, he alleged Federal Security Service agents co-ordinated apartment block bombings in Russia that killed more than 300 people.

He was one of several former Russian intelligence officers to accuse Moscow of being behind the dioxin poisoning of Ukrainian President Viktor Yushchenko during his 2004 election campaign.

Kremlin critics claim poisoning, which is very difficult to prove, is a common Soviet-era practice that seems to have reappeared since Putin, an ex-KGB officer, became president.