Litvinenko fell ill after meeting two Russian contacts, sparking speculation that he had been poisoned by the Kremlin.
Poisoning unexplained
Doctors earlier ruled out an initial theory that the heavy metal thallium was responsible and said radioactivity was "unlikely".
They also dismissed a report that three unidentified objects had been found in his intestines.
In Helsinki, where President Vladimir Putin is to attend an EU-Russia summit on Friday, a source in the Russian delegation said late Thursday, just prior to news of the death: "Of course it's a human tragedy. A person was poisoned. But the accusations against the Kremlin are so incredible, so nonsense-like, so silly, that the president cannot comment."
Oleg Gordievsky, a former colonel in the KGB who defected to Britain in the 1980s, said there was "no doubt" that the Russian secret service was responsible for Litvinenko's death.
Gordievsky told the BBC that he was "very angry that the Russian security service was ... so evil," and described Putin as "an international terrorist".
Vocal critic
Politkovskaya - also a vocal critic of Putin - was gunned down at her Moscow flat on October 7.
One of the two men Litvinenko met on the day he fell ill has told the Daily Telegraph newspaper that he feels he is being set up.
Andrei Lugovoi said that he had "the feeling that someone is trying to set me up as the fall guy."
The two men, along with Dmitry Kovtun, one of Lugovoi's business associates, met for between 20 minutes and a half hour, during which time they mainly discussed a business deal, he said.
"We had nothing to eat, but I cannot remember if he had tea or not ... But I'm 100 per cent certain that we did not offer him any because I was in a hurry."
Litvinenko fled Russia and was granted asylum in Britain after accusing the FSB - the Russian successor to the KGB - of plotting to kill the exiled Russian businessman Boris Berezovsky.
British police have said they are investigating what they called an "unexplained" death. Anti-terrorism police were called in last week after doctors determined that his illness was caused by poison.
Alexander Goldfarb, another friend of Litvinenko, told the BBC he was "confident that the ... police will get to the bottom of this."
