The Russian oligarch Boris Berezovsky has been found dead aged 67.
His death was first reported in a Facebook post from his son-in-law Igor Schuppe, according to Russia Today.
Thames Valley Police says they have launched an investigation into the death of a 67-year-old man in Ascot, Berkshire, which they describe as "unexplained". The area around the property has been cordoned off.
Lord Tim Bell, the advertising and PR executive, said Berezovsky's body found by his bodyguard. News reports said his body was in the bath.
Lord Bell, a close friend of Berezovsky told The Telegraph: "He was a very nice man, very kind to me and to the people around him."
Lord Bell said he had been told of the death on Saturday afternoon by Berezovsky's private lawyer.
Damian Kudriavtsev, the former CEO of Kommersant Publishing House, wrote on Twitter in Russian that his friend had died at 11am, London time, and that there were no signs of a violent death. The circumstances are unknown, but have been widely reported to be suicide.
A South Central Ambulance Service spokeswoman said: "We were called at 3.18pm by a caller who was concerned about the welfare of a gentleman at an address in Ascot.
"We sent a number of ambulance officers and an ambulance to the address. The 67-year-old male was confirmed deceased at the scene."
James Nixey, of thinktank Chatham House's Russian programme, told Sky News: "He is the most virulently anti Kremlin, anti Putin of the oligarchs.
"He was certainly willing to spend his money, what little he had left, in an attempt to use it to end the current regime in Russia.
"Putin put in a request for extradition. This was denied on the basis that he wouldn't receive a proper trial in Russia. But he was a wanted man in Russia.
"He lived in a heavily guarded flat in Mayfair. He had bodyguards, there were attempts on his life that even the security service in the UK had warned him about.
"It's certainly not the first case of Russians and people from the former Soviet Union, more broadly, who have been involved in difficult, embarrassing disputes with the Kremlin, to have died in relatively mysterious circumstances, perhaps before you might expect their natural life to end.
"Information is not always made public so I don't suppose we will find any answers particularly quickly."
Berezovsky is believed to have been possibly been in financial difficulties, after he ran up more than tens of millions of pounds in legal bills in less than two years in a court case against Chelsea owner Roman Abramovich.
He also built up legal costs totalling more than £250,000 just two months after becoming embroiled in a case at the same court with his former partner, Elena Gorbunova, who sat at Berezovsky's side throughout much of his battle with Abramovich.
Earlier this week, the Daily Telegraph reported that the Russian oligarch would attempt to sell Red Lenin, an Andy Warhol screen print valued at between £30,000 and £50,000 by Christie's.
Born in 1946, Berezovsky made his fortune in Russia in the 1990s, gaining control over various state assets, including the country's main television channel, Channel One.
In 1997 Forbes magazine estimated Berezovsky's wealth at £1.9bn.
During Boris Yeltsin's presidency, whe was deputy secretary of Russia's security council but following the Russian presidential election in March 2000, Berezovsky went into opposition and resigned from the Duma, moving the the UK in the wake of allegations of money laundering.
He has been on Russia’s wanted list since 2001 on charges of fraud, money-laundering and plots to overthrow the government, being given a total of 19 years in prison by various Russian courts in absentia.
In 2003, the UK granted Berezovsky political asylum, on the grounds that he would not receive a fair trial in Russia.
In 2012 Berezovsky lost a High Court case he brought against Roman Abramovich in London over the ownership of Sibneft.
Berezovsky claimed more than £3 billion damages after accusing Abramovich of blackmail, breach of trust and breach of contract. Abramovich denied the allegations and denied that Berezovsky was entitled to damages.
Mrs Justice Gloster ruled in Abramovich's favour when delivering a summary judgment in September 2012.
She said she found Berezovsky to be "an unimpressive, and inherently unreliable, witness, who regarded truth as a transitory, flexible concept, which could be moulded to suit his current purposes.
"At times, the evidence which he gave was deliberately dishonest; sometimes he was clearly making his evidence up as he went along in response to the perceived difficulty in answering the questions in a manner consistent with his case; at other times, I gained the impression that he was not necessarily being deliberately dishonest, but had deluded himself into believing his own version of events.