Another friend, Alexander Goldfarb, who organized Litvinenko's emigration to Britain, said FSB agents had threatened against him in the past.

"He looks like a ghost," Goldfarb said. "He's a very fit man, he never smoked, he never drank, he would run five miles a day, but now he has lost all his hair, he has inflammation in the throat, so he cannot swallow."

Russian authorities did not immediately comment on the allegations.

Litvinenko joined the KGB in 1988 and rose to the rank of colonel in its successor, the Federal Security Service, known as the FSB. He began specializing in terrorism and organized crime in 1991, and was transferred to the FSB's most secretive department on criminal organizations in 1997.

He fled Russia and claimed asylum in Britain in November 2000, two years after publicly accusing his FSB superiors of ordering him to kill Berezovsky, at the time a powerful Kremlin insider. Berezovsky said Sunday that Litvinenko fell out with his superiors after he exposed corruption within FSB ranks.

Before he left Russia, Litvinenko was jailed for nine months awaiting trial on charges of abusing his office; he was acquitted.

Kremlin critics claim poisoning - which is extremely hard to prove - is a common Soviet-era practice that seems to have reappeared since Putin, an ex-KGB officer, became president.

"It is not a secret that poisoning has become some kind of a trademark of a secret war in Russia," Alexander Golts, political commentator with the Russian news Web site Ezhenedelny Zhurnal, told the Associated Press. "I will not take the risk of accusing the government ... but certain groups have quite overtly been eliminating people they disliked through poisoning.

"It is absolutely obvious that this story with Litvinenko fits very well into the overall picture of power struggle in Russia," he said.

Politkovskaya, who had written critically about abuses by Russian forces fighting separatists in Chechnya, fell seriously ill after drinking tea on a flight from Moscow to southern Russia in 2004 during the school hostage crisis in Beslan. Colleagues say she was poisoned.

Yuri Shchekochikhin, a liberal Russian lawmaker and journalist who crusaded against corruption, died in July 2003 after apparently suffering a severe allergic reaction. Colleagues suspect he was poisoned, probably in connection with his reports on a case involving customs officials and allegations that a furniture store had evaded millions of dollars in import duties.

Yushchenko, the Ukrainian president, had his face badly disfigured by what doctors said was dioxin poisoning.

In one of the most notable Cold War assassinations, the Bulgarian dissident Georgi Markov was killed in 1978 with a poison dart concealed in an umbrella. British investigators long have suspected Bulgarian agents in the slaying.

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