Nov. 24 (Bloomberg) -- Alexander Litvinenko, a former Russian spy who said he was poisoned in London because of his opposition to the Kremlin, has died in a hospital.

Before his death Litvinenko alleged he was targeted because of his fierce criticism of the government of Russian President Vladimir Putin. The Kremlin and the Russian Foreign Intelligence Service, or SVR, have denied any involvement.

``Death is always a tragedy,'' Kremlin spokesman Dmitry Peskov said by telephone today. ``Now it's time for the relevant authorities in Britain, where Mr. Litvinenko lived, to investigate the case.''

Every medical avenue was explored to establish the cause of Litvinenko's illness and it remains a mystery, Jim Down, a spokesman for University College Hospital in London, said late yesterday. Litvinenko, 43, died at 9:21 p.m. on Nov. 23, he said.

U.K. police are treating his death as ``unexplained'' and haven't yet begun a murder investigation. ``Inquiries continue into the circumstances surrounding how Mr. Litvinenko became unwell,'' Scotland Yard said in an e-mailed statement. The U.K.'s top counter-terrorism officer, Peter Clarke, is leading the investigation.

Litvinenko himself was in no doubt that he was poisoned by Russian enemies. ``The bastards got me but they won't get everybody,'' he said shortly before he stopped being able to communicate on Nov. 21, his friend Andrei Nekrasov told the Times. Before he died the ex-spy looked ``like a survivor of the Nazi concentration camps,'' Nekrasov said.

`Shocked and Horrified'

Photographs of Litvinenko released Nov. 20 showed him lying in a hospital bed looking frail. His hair had fallen out.

His sickness wasn't due to a heavy metal such as thallium, as doctors initially thought, Geoff Bellingan, director of critical care at the hospital, said yesterday. It was also ``unlikely'' that he had suffered radiation poisoning, the doctor said. ``Despite extensive tests we are still unclear as to the cause of his condition,'' Bellingan said.

Litvinenko's family are ``all shocked and horrified at this terrible crime,'' family spokesman Alex Goldfarb said outside the hospital after the former spy's death.

Litvinenko, 43, a former lieutenant colonel in Russia's FSB, or Federal Security Service, the successor to the KGB, claimed asylum in the U.K. six years ago and became an outspoken critic of the Kremlin over issues including the conflict in Chechnya.

Russian Journalist Killed

He had been investigating the killing of Russian journalist Anna Politkovskaya, 48, another prominent critic of Russian President Vladimir Putin and his policy in Chechnya. Politkovskaya was shot at her Moscow apartment building Oct. 7.

He first reported feeling ill on Nov. 1 after meeting with a friend at a Japanese sushi restaurant, Itsu, in London's Piccadilly.

Mario Scaramella, an Italian, said at a Nov. 21 news conference in Rome that he was the friend. Scaramella said he and Litvinenko had both received e-mailed threats days before the poisoning.

Shortly before the restaurant meeting Litvinenko met two Russian men, including a former KGB officer whom he knew, for tea at a hotel in central London, Goldfarb said.

Litvinenko's death has echoes of Cold War assassinations such as that of Georgi Markov, a Bulgarian dissident who died after being spiked with a poison-tipped umbrella on a London street in 1978.

To contact the reporter on this story: Nick Allen in London at nallen14@bloomberg.net

Last Updated: November 24, 2006 05:59 EST