Russian oligarch Boris Berezovsky has died at the age of 67.
The exiled businessman, who last year lost a multibillion-pound High Court fight with Chelsea Football Club owner Roman Abramovich, was found dead at his home in Berkshire.
Lord Tim Bell, the advertising and PR executive, confirmed his death.
A spokesman for his company, Bell Pottinger, said Berezovsky was found at his home in Ascot, Berkshire.
ITV News Correspondent Romilly Weeks reports:
Berezovsky was part of Boris Yeltsin's inner circle in the latter years of his presidency, and became deputy secretary of Russia's security council.
He acquired his fortune in the 1990s after the privatisation of state assets following the collapse of Soviet Communism.
The businessman survived a number of assassination attempts, including a bomb in his car that decapitated his chauffeur.
Berezovsky emigrated to the UK in 2000 after falling out with President Vladimir Putin.
In fear of his life, he sought political asylum and moved to the South East of England, buying upmarket properties in Knightsbridge and Berkshire.
Berezovsky ran up more than tens of millions of pounds in legal bills in less than two years after the court battle with 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 Mr 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.
The artwork sold on Wednesday for £133,875 including the buyer's premium, according to the auction house's website.
Last year, Mrs Justice Gloster dismissed a series of claims by Berezovsky - who wanted more than £3 billion damages - relating to deals done in Russia with Mr Abramovich.
In October, the judge was told by lawyers that Berezovsky had agreed to pay £35 million towards Mr Abramovich's legal costs in the wake of her ruling.
She was given no detail of the amount of costs run up by Berezovsky.
But legal magazine The Lawyer said the case was thought to have generated "total fees" of more than £100 million.
Berezovsky had sued and accused Mr Abramovich of blackmail, breach of trust and breach of contract.
He said the billionaire Russian businessman had "intimidated" him into selling shares in a Russian oil company at a fraction of their value and broken a promise made during a deal relating to a Russian aluminium company.
Mr Abramovich said the claims had "no merit".
Mrs Justice Gloster ruled in Mr Abramovich's favour in August following a trial - staged in London between October 2011 and January - at which both men gave evidence.