Russian officials have demanded a further $10bn (£5.6bn) in back taxes from oil giant Yukos, according to the Reuters news agency.
Yukos is Russia's biggest oil producer
The demand takes Yukos' total tax arrears to $17bn, Reuters reported, citing unnamed sources.
Yukos separately said its key Siberian oil unit, Yuganskneftegas, had been issued with a $3.4bn tax demand.
A Yukos spokesman said it would challenge the tax claim against Yuganskneftegas in court.
"We consider that we have acted within the bounds of the law," he said.
The Russian authorities' move is the latest twist in a year-long crackdown on Yukos, the country's biggest oil producer.
The government's aggressive scrutiny of Yukos' tax affairs is widely seen as a politically-motivated response to the political ambitions of its former chief executive, Mikhail Khodorkovsky.
Mr Khodorkovsky, who had funded liberal opposition groups, was arrested in October last year on fraud and tax evasion charges.
Even before Monday's fresh $10bn demand, it was widely thought that Yukos would have little option but to sell Yuganskneftegas, which accounts for more than 60% of its output, in order to settle its tax bill.
Analysts believe that the company is likely to be bought up by a government-backed firm, effectively bringing a large chunk of Russia's lucrative oil and gas industry back under state control.