The majority owner of embattled Russian oil firm Yukos has sued the Russian government for $28.3bn (£15.2bn).
Yukos is now worth a fraction of its former value
The Kremlin last year seized and sold Yukos' main production arm, Yugansk, to state-run oil group Rosneft for $9.3bn to offset a massive back tax bill.
Group Menatep, the Gibraltar-based holding company which controls 51% of Yukos, says this was illegal.
Menatep has already asked Rosneft to repay a $900m loan that Yugansk had secured on its assets.
The Russian government's argument for selling Yuganskneftegaz - the unit's full name - was that Yukos owed more than $27bn in back taxes for the years from 2000 onwards.
It accused the firm of using a web of offshore firms to avoid its tax liabilities, and the courts sent in bailiffs to freeze Yukos accounts and seize Yugansk.
But critics say the sell-off, and the assault on Yukos' finances, are part of an attempt to bring the energy industry back under state control.
According to Menatep, the government's actions were contrary to the 1994 Energy Charter Treaty, which was designed to regulate disagreements over energy investments.
"We have warned the Russian government about their continuing attacks against Yukos, its personnel and its shareholders and we have warned any buyer of Yuganskneftegaz that they would face a lifetime of litigation," said Tim Osborne, a director of Group Menatep.
"The time for warning is over and actions to recover the value of our losses begin in earnest today."
Menatep said the value of its Yukos shareholding had gone from $17.8bn to "virtually nothing" since 2003 as a result of the Russian government's action, as its shares have fallen 97%.
According to its Paris lawyer, Emmanuel Gaillard of Shearman and Sterling, the overall claim is based on that figure, with a 60% addition for the share gains that could have accrued since then.
Arbitration of the lawsuit could take place in Stockholm or The Hague, Mr Gaillard said.
While Russia has signed the Charter, it has never ratified it - which some experts say could make it difficult for Menatep to press its case.
But Mr Gaillard told BBC News that the Charter came into effect on signature, not ratification.
"Russia has said in the past that it is bound by it, so as to attract foreign investors," he said.
Yukos is still waiting to see what will happen to its filing in a US court for bankruptcy protection.
It took the action to try to prevent the forced sale of Yugansk - first to a little-known shell company, which in turn was bought by Rosneft.
Yukos claims its downfall was punishment for the political ambitions of its founder Mikhail Khodorkovsky.
Mr Khodorkovsky, currently facing fraud and tax evasion charges of his own, was one of the founders of Menatep.
He has since signed over his shareholding to one of his fellow investors.