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)