Boris Berezovsky has been found dead at his UK home in Surrey.
The exiled Russian tycoon had survived numerous assassination attempts in the past but the circumstances of his death are currently unknown.
The 67-year-old was a wanted man in Russia in connection with the Aeroflot fraud case.
The former government official was also a staunch critic of Vladimir Putin.
He resigned from the Duma in 2000 saying he did not “want to be involved in the country’s ruin and the restoration of an authoritarian regime.”
He moved to the UK in the same year after seeing his fortunes decline under Putin’s presidency.
In 2011 he had also been in a high profile legal dispute with Chelsea Football Club owner, Roman Abramovich.
He claimed the fellow Russian oligarch had intimidated him into selling shares in oil giant Sibneft, and sought £3bn (3.5 bn euros) in compensation.
The high court found in favour of Abramovich.