Police said the death, at Berezovsky's home near London, was officially being treated as unexplained, the BBC reported.
Berezovsky was among a small group of rich men who dominated post-Soviet Russia. He was a close ally of Boris Yeltsin -- who served from 1991-99 as the first president of the Russian Federation -- and helped install Vladimir Putin as president.
He later became a Putin enemy and called for Putin's ouster, The New York Times reported.
Berezovsky, who moved to Britain in 2000, lost a $5.1 billion lawsuit in 2012 against another Russian oligarch, Chelsea Football Club owner Roman Abramovich, and ended up with $250 million in legal and other costs, the Times said.
The dispute involved the sale of shares in a Russian oil company. Berezovsky claimed he had been strong-armed into selling the shares for a "fraction of their true worth," but a British court ruled against Berezovsky, with a judge calling him an "inherently unreliable" witness, the BBC said.
Citing sources, the BBC said Berezovsky had been depressed after losing the lawsuit and was feeling financial pressure.
The Times said Berezovsky recently had begun to sell off his personal assets, including an Andy Warhol painting and a yacht.
The Times said Berezovsky's death was first reported on Facebook by his son-in-law, and confirmed by a lawyer, Alexander Dobrovinsky.
Dobrovinsky said Berezovsky may have committed suicide.
Berezovsky was a leading Soviet mathematician who went into business and figured out how to skim profits from Russian's largest state-owned carmaker, the Times said.
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