Alexander Litvinenko was admitted to a London hospital two weeks ago and diagnosed with acute poisoning believed to have been caused by a potentially deadly concentration in his blood of thallium, a chemical used in rat poison and insecticides. He reportedly felt sick after meeting over lunch with an Italian contact, who had said he would like to hand over some documents concerning the recent murder in Moscow of investigative journalist Anna Politkovskaya.
The reporter, known for her harshly critical articles about the Kremlin's policies in Chechnya, was shot dead in a contact-style killing on October 7 this year.
A Scotland Yard spokesperson said Sunday, "Officers from the specialist crime directorate are investigating a suspicious poisoning. No arrests have been made."
The former secret agent fled Russia after three criminal cases had been opened against him in 2000, and was granted political asylum by the British government. He is believed to be a close associate of exiled Russian tycoon Boris Berezovsky, who also moved to the UK to escape prosecution in his home country.