British police are investigating a suspected plot to kill a former Russian spy by poisoning him in a London sushi restaurant.
Exiled agent Alexander Litvinenko, an outspoken critic of Russian President Vladimir Putin, is in a 'serious but stable' condition in hospital, a police spokesman said on Sunday.
Litvinenko said he fell ill after meeting a contact while investigating the Moscow murder of Russian journalist Anna Politkovskaya.
'I do feel very bad. I've never felt like this before - like my life is hanging on the ropes,' Litvinenko said from his hospital bed.
After visiting him on Sunday, close friend Alex Goldfarb said: 'The doctors say that the next four weeks are critical. His chances for survival are 50-50. He looks like an old man and he has lost all his hair.'
Scotland Yard police headquarters refused to go into details but a spokeswoman said: 'Officers from the specialist crime directorate are investigating a suspicious poisoning. No arrests have been made. Inquiries are continuing.'
She said: 'His condition is serious but stable.'
Media reports said Litvinenko had been poisoned with the deadly chemical thallium which is used in rat poisons and insecticides. It has earned the macabre nickname 'inheritance powder'.
Thallium attacks the nervous system lungs, heart, liver and kidneys. The colourless, odourless toxin results in hair loss, vomiting and diarrhoea. One gram can be enough to kill.
Litvinenko, a former colonel in the Russian secret service, fell ill earlier this month after having lunch in a sushi bar with a mysterious contact known only as Mario.
'I ordered lunch but he ate nothing. He appeared to be very nervous. He handed me a four-page document which he said he wanted me to read right away,' Litvinenko told the paper.
'It contained a list of names of people, including FSB (Federal Security Service) officers, who were purported to be connected with the journalist's murder,' he added.
Litvinenko said: 'They probably thought I would be dead from heart failure by the third day.'
Politkovskaya, who had criticised Kremlin policy in the troubled Chechnya region, was shot dead by an unknown gunman in her apartment building on 7 October.
Litvinenko's case echoed the poisoning of Ukraine's Viktor Yushchenko, who fell ill after dining with security service leaders while he was a presidential candidate in 2004. Doctors found he had been poisoned with dioxin.
Bulgarian dissident Georgi Markov was murdered in London with a poison-tipped umbrella in 1978, at the height of the Cold War.
