Both Lugovoi and the Kremlin denied the accusations, with the former instead claiming that Berezovsky -- whom Russia consistently sought to extradite on a wide variety of criminal charges -- engineered his friend's death as a way of embarrassing the Kremlin and buttressing his refugee status.

Berezovsky, who considered Litvinenko a close friend, consistently denied the allegations. In 2010, he won a libel case against Kremlin-owned broadcaster All-Russian State Television and Radio Broadcasting, which aired a show in which it was suggested he was behind the former agent's poisoning.

Berezovsky recently has made headlines for costly legal battles, which have dealt serious blows to his finances.

Last year, the Russian business magnate was ordered to pay 35 million pounds ($53.3 million) in legal costs to fellow Russian Roman Abramovich, the billionaire owner of Chelsea Football Club, after losing a multimillion-dollar legal battle against him.

Berezovsky had claimed that Abramovich cheated him out of his stakes in the oil group Sibneft, arguing that he blackmailed him into selling the stakes vastly beneath their true worth after he lost Putin's good graces. But a judge threw out the case in August, ruling that Berezovsky was a dishonest and unreliable witness.

It also recently emerged that Berezosky ran up legal bills totaling more than 250,000 pounds in a case against his former partner, Elena Gorbunova, with whom he had two children and who claimed the businessman owed her millions.

Earlier this week, The Times of London newspaper reported that Berezovsky was selling property -- including an Andy Warhol portrait of the former Soviet Union leader Vladimir Lenin -- to settle his debts and pay expenses owed to lawyers.

The Russian president's spokesman, Dmitry Peskov, said in a telephone interview on state television that Berezovsky had sent a letter to Putin about two months ago asking to be allowed to return to Russia. In the letter, Berezovsky acknowledged having made many mistakes, Peskov said.

Peskov said he did not know how Putin reacted to news of the death.

"But you can say that information about the death of someone, no matter who he was, cannot elicit positive emotions," the spokesman said.

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