The tycoon, who left Russia for Britain in 2000 after falling out with the Kremlin, died at his home in Surrey on Saturday, according to Russian news agencies.
Andrei Sidelnikov, a prominent Russian opposition figure and friend of Mr Berezovsky, said: “I can confirm he died in his home. I’ve known him for a long, long time, we have spent a lot of time together. I am shocked. It is the end of an epoch.”
Mr Berezovsky, 67, lost a court battle against the Russian billionaire Roman Abramovich last year.
It is the end of an epoch. Andrei Sidelnikov, Russian opposition figure
He alleged that Mr Abramovich had “intimidated” him into selling his stake in the Russian oil company Sibneft at a fraction of its true value.
He was suing for breach of trust and breach of contract and was claiming for more than £3 billion in damages from Mr Abramovich.
Today the Russian Interfax news agency said that a relative had confirmed Mr Berezovsky’s death.