Former Russian spy Alexander Litvinenko who was poisoned by an unknown toxic substance three weeks ago died on Thursday at the intensive care unit of London's University College Hospital (UCH).

The UCH said in a statement that doctors couldn't determine the cause of the death of Litvinenko, who has been hospitalized in the UCH since last Friday.

"He was seriously ill when he was admitted to UCH on Friday, Nov. 17, and the medical team at the hospital did everything possible to save his life," said the statement.

"Every avenue was explored to establish the cause of his condition and the matter is now an ongoing investigation being dealt with by detectives from New Scotland Yard," it noted.

UCB doctors ruled out the possibility that Litvinenko, 43, had been poisoned by thallium and radiation, two deadly toxins once believed likely causes of his illness. Photos released earlier by the hospital showed that Litvinenko's hair had fallen out and his skin became swollen.

Litvinenko presumed that he was poisoned on Nov. 1 at a sushi bar in west London where two Russians offered him information relating to the Oct. 7 killing of Russian investigative reporter Anna Politkovskaya. Litvinenko had been investigating the woman reporter's murder.

Litvinenko and other Russian exiles in Britain accused the Kremlin of orchestrating his poisoning, an accusation vehemently denied by Moscow.

Litvinenko joined the Soviet spy agency KGB in 1988 and then became a colonel of its successor, the Russian Federal Security Service (FSB). In 1998 he became an open critic of the FSB and was arrested several times. He fled to Britain with his wife and son in November 2000 and was granted asylum.

Source: Xinhua