Boris Berezovsky, the exiled Russian oligarch and long-time opponent of the Kremlin, has died in Britain at the age of 67, his spokesman said on Saturday.
British media said Berezovsky was found dead in the bath of his home near London but there was no immediate confirmation of the circumstances of his death.
The tycoon was involved in a bitter multi-million pound legal battle with fellow tycoon and Chelsea Football Club owner Roman Abramovich last year.
"Yes, he is dead. It was confirmed to me by his private lawyer this afternoon," Berezovsky's spokesman Tim Bell told AFP by telephone.
Bell did not give any further details but said he would issue a statement later on behalf of Berezovsky's family.
There was no immediate comment from British police.
A Russian lawyer, Alexander Dobrovinsky, said Berezovsky had committed suicide, although there was no confirmation from other sources.
"I received a call from London saying that Berezovsky had committed suicide," he told Russia 24 news channel.
"Recently Berezovsky had been in a horrible state, very depressed. He had nothing but debts, he was practically ruined, he was selling his paintings," Dobrovinsky said.
Berezovsky had recently written to Russian President Vladimir Putin to ask for a pardon and to say that he wanted to return to Russia, Russian news agencies cited Putin's spokesman Dmitry Peskov as saying.
Berezovsky settled in Britain more than a decade ago after going into "self-imposed exile" and has already been convicted and jailed in absentia by Russian courts on embezzlement charges.
Berezovsky was last year seeking more than £3 billion ($4.75 billion, 3.8 billion euros) in damages from Abramovich after accusing his rival of blackmail, breach of trust and breach of contract.
He lost the case and subsequently agreed to pay Abramovich £35 million ($56 million) in legal costs.
But his biggest battles were with the Kremlin.
He was at the centre of a group of anti-Putin exiles in London who included Russian dissident Alexander Litvinenko, who died of poisoning by radioactive polonium in November 2006.
Just two weeks ago, Andrei Lugovoi, a former FSB agent regarded by Britain as the prime suspect in Litvinenko's murder, accused Berezovsky of being behind the gruesome murder.
Berezovky's death comes as Britain and Russia are trying to mend their relations, which were chilled in the wake of the Litvinenko killing.
His career traced the arc of Russian society from the dawn of free enterprise in the Soviet Union's dying days to the oligarch-dominated 1990s, then the return of state control in the first decade of the new millennium.
Born on January 23, 1946, Berezovsky graduated from a Moscow forestry institute then spent nearly two decades as a quiet academic before becoming a car dealer in the late 1980s and a billionaire oligarch by the following decade.
Putin began his presidency in 2000 by warning that the heyday of super-rich powerbrokers like Berezovsky was over. Berezovsky fled into exile that November, just in time to escape arrest on fraud charges.
In London, he became the Kremlin's greatest nemesis, mockingly defying years of attempts to extradite him. He emerged from an extradition hearing in 2003 wearing a Putin mask.