University College Hospital said Litvinenko, who had been a public critic of the Russian government, passed away on the night of November 23 in the intensive-care unit.
"We are sorry to announce that Aleksandr Litvinenko died at University College Hospital at 9:21 [p.m.] on the 23rd of November, 2006" hospital spokesman Jim Brown said. "He was seriously ill when he was admitted to UCH on Friday, November 17, and the medical team at the hospital did everything possible to save his life."
London's Metropolitan Police said in a statement that investigators are treating the matter as an "unexplained death."
Doctors said they still have not determined the exact form of Litvinenko's poisoning. Doctors have ruled out suggestions it may have been caused by a heavy metal such as thallium or by radiation.
"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," Down said. "Because of this, we will not be commenting any further."
Litvinenko, 43, was said to have recently been pursuing information about the unsolved murder in October of Russian investigative journalist Anna Politkovskaya. Litvinenko's supporters have accused the Kremlin of being behind a plot to poison him.
The Russian government has denied playing any role in the case.
(compiled from agency reports)
The Kremlin's Fallen Foes
Mourners in Moscow mark the 40th day after the killing of investigative journalist Anna Politkovsksya on November 15 (TASS)
DANGEROUS DISSENT. A surprising number of vocal critics of the policies of Russian President Vladimir Putin have been killed or have died mysteriously in recent years. Although the Russian government has denied any involvement in any of the cases, some Kremlin watchers have begun speaking of a clandestine campaign to eliminate dissent.
April 17, 2003: Sergei Yushenkov, veteran liberal politician, Duma member, and leader of a staunchly anti-Kremlin party, is shot dead in Moscow.
July 3, 2003: Yury Shchekochikhin , liberal lawmaker and investigative journalist, dies of a mysterious allergic reaction. Many believe it was a case of deliberate poisoning, but the incident was never investigated as a murder.
February 13, 2004: Former acting Chechen President Zelimkhan Yandarbiyev is killed in an explosion in Doha, Qatar. Two Russian security-service agents are later convicted of carrying out the killing.
September 2, 2004: Investigative journalist Anna Politkovskaya falls ill mysteriously on a plane bound for the North Caucasus. Politkovskaya was heading to Beslan, North Ossetia, in a bid to negotiate the release of schoolchildren being held hostage there by Chechen militants.
December 2004: Former Ukrainian Prime Minister Viktor Yushchenko , running for president as a pro-Western candidate against Prime Minister Viktor Yanukovych, is poisoned. He recovers and goes on to win the presidency, although his poisoning remains a mystery.
October 7, 2006: Investigative journalist and vocal critic of Russian policies in the North Caucasus Anna Politkovskaya is gunned down in Moscow.
November 23, 2006: Former Federal Security Service agent Aleksandr Litvinenko, a vocal critic of Russia's secret services, dies of a mysterious poisoning in London.