The Interfax news agency cited a relative whom it did not name as saying Berezovsky, 67, had died. Other agencies said he was discovered in the bath of his home on Saturday.
Thames Valley police say his death is being treated as unexplained.
Police would not directly identify Berezovsky, but when asked Saturday about him by name they read a statement saying they are investigating the death of a 67-year-old man at a property in Ascot, a town 40 kilometres west of London.
Condolences were expressed in postings Saturday to the Facebook page of Berezovsky's son-in-law, Egor Schuppe, shortly after the younger man posted a brief entry that only said, "died, Boris Berezovsky."
Berezovsky built his fortune in the years after the fall of the Soviet Union when the new Russian leadership was selling off state enterprises and he became part of then-president Boris Yeltsin's inner circle.
He left Russia for England in 2000 after a falling-out with President Vladimir Putin and had an estate in Surrey.
Last year, the exiled businessman was ordered to pay £35 million in legal costs to fellow Russian Roman Abramovich after losing a multimillion-dollar legal battle against the billionaire owner of Chelsea Football Club.
Berezovsky had claimed that Abramovich cheated him out of his stakes in the oil group Sibneft, arguing that he blackmailed him into selling the stakes vastly beneath their true worth after he lost Putin's good graces. But a judge threw out the case in August, ruling that Berezovsky was a dishonest and unreliable witness.