A BBC investigation has uncovered new evidence about the corrupt deals that made Roman Abramovich's fortune.
The Chelsea owner made billions after buying an oil company from the Russian government in a rigged auction in 1995.
Mr Abramovich paid around $250m (£190m) for Sibneft, before selling it back to the Russian government for $13bn in 2005.
His lawyers say there is no basis for alleging he has amassed very substantial wealth through criminality.
The Russian billionaire was sanctioned by the UK government last week because of his links to Russian President Vladimir Putin.