Russian oil company Yukos's core owners have written to President Putin threatening to sue Russia in international courts if officials refuse to start talks over its giant tax bill.
Yukos is facing collapse under $17.5 billion (€13.52 billion) in bills and fines for unpaid taxes, and main owner Mr Mikhail Khodorkovsky is on trial for fraud and tax evasion.
Holding company Menatep, through which Mr Khodorkovsky and associates control Yukos, has threatened to sue Russia under an energy charter treaty that protects energy investments but which Moscow has not yet ratified.
"This is a more insistent invitation to dialogue," said Menatep spokesman Mr Yuri Kotler.
"If such a case is brought, Russia is legally obliged to attend the hearing. It's a separate matter that Russia could say that since it has not ratified the treaty it does not recognise the action.
"In that case, it is not clear what Russia would do as it would be an important moment for its image." Menatep owns 60 per cent of Yukos.
