Foreign bankers have warned Yukos they may call in a $1bn (£600m) loan in a move the oil giant yesterday blamed on the Russian government's demands for a $3.5bn tax payment.
Shares in the Russian oil group dropped 9.5 per cent after it issued a statement saying its lenders considered a "potential event of default has occurred".
Bruce Misamore, Yukos' financial officer, said: "This notice by the lending group is a direct result of the actions of the tax ministry in its attempt to claim taxes, interest, fines and penalties from Yukos, which are not due."
The tax claim is part of a broader judicial investigation into the affairs of the oil group whose main owner, Mikhail Khodorkovsky, has been in jail since October pending trial on charges of fraud and tax evasion.
Standard & Poor's slashed its Yukos rating by five notches to CCC last week, citing a threat of default posed by a court ban on asset disposals as the company fights the $3.5bn back-tax bill in the courts.
Bankers said that while the loan contained no ratings triggers that would make it immediately payable, members of the syndicate, which includes Citigroup, HBOS, Commerzbank and ING among others, were considering their positions on an ongoing basis. "Liquidity is the main problem but banks' main concern is that as Yukos' assets are frozen they cannot be offered up as security against the loan," one banker said.
Analysts said Western banks were finding it increasingly difficult to believe Kremlin promises that Yukos would survive as a company.