British media report that Aleksander Litvinenko was taken ill on November 1 while investigating the recent murder of Russian reporter Anna Politkovskaya, who had criticized Kremlin policy on the rebellion in Chechnya.
Litvinenko defected to Britain six years ago and was granted British citizenship last month.
One of the highest-profile defectors from Russia's Federal Security Service, Litvinenko is on a wanted list in Moscow, where he has made powerful enemies with his severe criticism of President Vladimir Putin.
Remembering Anna Politkovskaya
Anna Politkovskaya at RFE/RL in July 2006 (RFE/RL)
A BRAVE VOICE SILENCED. Prominent Russian journalist Anna Politkovskaya was killed on October 7 in her Moscow apartment building. An outspoken critic of the Kremlin, she was best-known for her reporting on the conflict in Chechnya. Former Soviet President Gorbachev called her slaying "a true political homicide, a vendetta."