Alexander Litvinenko, a former Russian intelligence officer and a close associate of Russia's fugitive oligarch Boris Berezovsky, was admitted to a London hospital three weeks ago and initially diagnosed with acute poisoning.
Western media have circulated the deathbed note of Litvinenko, known as a fierce Kremlin critic, in which he accused Vladimir Putin of orchestrating his death.
The initial medical conclusion was that the patient had been poisoned with thallium, a chemical used in rat poison and insecticides. However, this diagnosis was later found to be false.
Doctors said Litvinenko lost all his hair and was unable to eat for 18 days. His bone marrow was badly damaged, depriving his body of white blood cells.
Litvinenko, who defected in 2000 and recently received British citizenship, said in an interview with the UK's Sunday Times newspaper he believed the poisoning was a murder plot to avenge his defection.
Russia's foreign intelligence service, the SVR, has denied any involvement in the poisoning, and President Vladimir Putin said Friday he regrets that people have used Litvinenko's death for political provocations.
Britain's Metropolitan Police earlier said they have called in experts to determine the potential presence of radioactive materials in Litvinenko's body, and have already discovered traces of radiation at a sushi restaurant where the ex-spy ate.
Polonium-210 is a highly radioactive isotope closely related chemically to bismuth and tellurium. Handling polonium-210 requires special equipment, and strict procedures. Direct damage occurs from energy absorption into tissues from alpha particles.
