Litvinenko, 43, a former officer of the Federal Security Service (FSB), formerly the KGB, 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.
The initial conclusion was based on a potentially deadly concentration in his blood of thallium, a chemical used in rat poison and insecticides.
But experts at London's University College Hospital failed to establish exactly what killed Litvinenko. He was transferred to intensive care after suffering heart failure in the last 48 hours.
Dr. Geoff Bellingan of the hospital said thallium was definitely not the cause of Litvinenko's critical condition.
"We are now convinced that the cause of Mr Litvinenko's condition was not a heavy metal such as thallium. Radiation poisoning is also unlikely," he said. "Despite extensive tests, we are still unclear as to the cause of his condition."
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 - a strong critic of the Russian government who defected in 2000 and recently received British citizenship - said in an interview with The 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 case.
Britain's Scotland Yard is investigating the alleged poisoning.
