Former Russian spy Alexander Litvinenko - poisoned in London three weeks ago - has taken a sudden turn for the worse and is now very seriously ill, the hospital treating him said on Thursday.
"There has been a major deterioration in Mr Litvinenko's condition overnight. He is now in a very serious condition and remains in intensive care," London's University College Hospital said.
Doctors have now ruled out poisoning by a heavy metal such as thallium or by radiation, said Dr Geoff Bellingan, clinical director of critical care at the hospital. The 41-year-old critic of Russian President Vladimir Putin has said he fell ill after meeting two Russians at a hotel.
Britain's anti-terrorism police were called in to investigate the case after doctors determined last week his illness was caused by poison. Bellingan told reporters shadows seen on Litvinenko's x-rays were believed to be caused by a chemical given as part of his treatment, not by poison, and therefore did not offer clues as to how he fell ill.
Litvinenko, now a British citizen, co-authored a book in 2002 entitled "Blowing up Russia: Terror from Within", in which he alleged Federal Security Service (FSB) agents co-ordinated apartment block bombings in Russia that killed more than 300 people in 1999. Officials blamed the bombings on Chechen rebels. He had been investigating the killing of Russian journalist Anna Politkovskaya, also a vocal critic of Putin, who was gunned down at her Moscow flat on October 7.