Boris Berezovsky, the exiled Russian oligarch and long-time opponent of the Kremlin, died at his home in Britain on Saturday in "unexplained" circumstances, his spokesman and police said.
Police said they had launched a full investigation into the 67-year-old's death in the well-heeled commuter town of Ascot, near London.
Berezovsky's lawyer Alexander Dobrovinsky said on Russian television the tycoon had committed suicide after suffering from weeks of depression over his huge debts, although another friend strongly denied this.
Berezovsky settled in Britain more than a decade ago after going into "self-imposed exile" where he became one of the most outspoken critics of President Vladimir Putin.
He was at the centre of a group of anti-Kremlin exiles in London who included Alexander Litvinenko, who died of poisoning by radioactive polonium in November 2006.
Putin's spokesman said however that Berezovsky had recently written to the Russian leader asking him for a pardon and permission to return home.
Berezovsky's spokesman in Britain said the oligarch had been found at his home in Ascot by his bodyguard, refusing to comment on media reports that he was found in his bath.
Police said the death was "unexplained" and they had cordoned off Berezovsky's home while an investigation was carried out.
"Thames Valley Police has launched an investigation into the death of a 67-year-old man at a property in Ascot, Berkshire. His death is currently being treated as unexplained and a full inquiry is under way," the force said in a statement.
A spokeswoman for the local ambulance service said they were called out at 3:18 pm (1518 GMT) "by a caller who was concerned about the welfare of a gentleman to an address in Ascot".
"We sent a number of ambulance officers and one ambulance to the address. The 67-year-old man was confirmed deceased at the scene," she said.
Berezovsky's colourful past is likely to prompt intense speculation about his death -- he was paranoid about plots against his life, and in 1995 he narrowly escaped an assassination attempt that decapitated his driver.
However, his lawyer told Russian state television that he had been informed by contacts in London that Berezovsky had killed himself.
"Berezovsky has been in a terrible state as of late. He was in debt. He felt destroyed," said Dobrovinsky. "He was forced to sell his paintings and other things."
The tycoon was involved in a bitter multi-million pound legal battle with fellow tycoon and Chelsea Football Club owner Roman Abramovich last year.
He sought 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.
Berezovsky also ran up £250,000 in costs in a legal battle with his former partner, Elena Gorbunova, with whom he had two children.
However, the oligarch's friend Demyan Kudryavtsev firmly denied that Berezovsky had killed himself.
"No! This is not so!" he was quoted as saying by the Prime news agency in Russia.
"Nobody knows this. There are no external signs of a suicide. There are no signs that he injected himself or swallowed any pills. No one knows why his heart stopped."
Born January 23, 1946, in Moscow, Berezovsky trained in forestry and worked as an academic for nearly two decades before becoming one of the super-rich oligarchs who dominated Russia in the 1990s.
Berezovsky's power peaked after he helped Boris Yeltsin become president in 1996, but his subsequent help for Putin to take over after Yeltsin proved his undoing.
The fast-talking Muscovite with a taste for the high life became a key target of Putin's crackdown on the oligarchs' political independence and he fled to Britain, where he won political asylum in 2003 and from where he became a vocal Kremlin critic.
When his friend Litvinenko died a gruesome death by radioactive poisoning in 2006, there were accusations from Moscow that Berezovsky was involved, something he strongly denied.
After news of Berezovsky's death on Saturday, Kremlin spokesman Dmitry Peskov said the oligarch had written to Putin a couple of months ago saying he wanted to go home.
"He asked Putin for forgiveness for his mistakes and asked him to obtain the opportunity to return to the motherland," the spokesman told Russian state television.