Boris Berezovsky, the exiled Russian oligarch and long-time opponent of the Kremlin, died at his home in Britain on Saturday aged 67 in unexplained circumstances, officials said.
British police launched a full investigation into the death at the mansion in the well-heeled commuter town of Ascot, near London, saying it was "currently being treated as unexplained".
Berezovsky's lawyer Alexander Dobrovinsky told Russian television the tycoon had committed suicide after suffering from weeks of depression over his huge debts, although another friend strongly denied this.
Berezovsky was one of handful of businessmen who made a fortune out of the privatisation of Russian state assets in the 1990s, and was close with President Vladimir Putin before he took office.
But Putin turned on the oligarchs when he came to power and in 2000 Berezovsky fled to Britain where he claimed political asylum and became one of the president's outspoken critics.
He was at the centre of a group of anti-Kremlin exiles in London which included Alexander Litvinenko, who died of poisoning by radioactive polonium in November 2006.
Paramedics were called to Berezovsky's estate in Ascot at 3.18pm (1518 GMT) on Saturday and the Russian was pronounced dead at the scene, the ambulance service said. Police were then called.
"His body was found by his bodyguard," said a spokesman for Berezovsky, refusing to comment on media reports that he was found in his bath.
Police cordoned off several roads around Berezovsky's home, set in a large gated park, as they pursued what a spokesman said would be a "full inquiry".
"The body is believed to be that of Russian national Boris Berezovsky. His death is still being treated as unexplained," said an updated statement late Saturday from Thames Valley Police.
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.
-- 'He felt destroyed' --
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."
Last year, the tycoon lost a bitter multi-million pound legal battle with fellow oligarch and Chelsea Football Club owner Roman Abramovich.
Berezovsky had 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 in an oil deal.
When he lost, he agreed to pay Abramovich £35 million ($56 million) in legal costs, although there is speculation that the full fees would come to far more than that.
Berezovsky's private life has also taken its toll on his finances, after a 2011 divorce with his second wife Galina Besharova that was dubbed the costliest in Britain, and a more recent legal wrangle with his partner Elena Gorbunova.
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 on January 23, 1946, in Moscow, Berezovsky trained in forestry and worked as an academic for nearly two decades before a car business and a string of shady deals made him a billionaire.
Berezovsky's power peaked after he helped Boris Yeltsin get re-elected as president in 1996, and he helped Putin take office four years later, although this would become 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.
When his friend Litvinenko died a gruesome death by radioactive poisoning, 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.
Asked for the president's reaction to Berezovsky's death, he said: "It is doubtful that news of the death of a person such as him can prompt any positive response."