A mystery company won an auction to buy the main production unit of Russian oil giant Yukos yesterday, adding a further twist to the country's most controversial forced sale.

Baikal Finance Group, an unknown company, outbid the previous favourites, state gas monopoly Gazprom, for the production unit, Yuganskneftegaz, in the state-managed sale.

Russia's State Property Fund said Baikal, named after a freshwater lake in Siberia, paid $9.37 billion dollars ( billion), the highest of four bids.

But just who runs Baikal, and where its finance comes from, remains a mystery.

The auction is the latest episode in a tale that has seen shares in Yukos, Russia's biggest oil company, tumble, taking the stock market down with it and helping to send world oil prices to record levels.

The story began in 2003 with the arrest of Yukos's chief executive, Mr Mikhail Khodorkovsky, on tax evasion and fraud charges.

Many analysts believe that the subsequent legal assault on Yukos, ending in a demand for $27.5 billion in back taxes, was inspired by the Kremlin to punish Mr Khodorkovsky for financing parties opposed to the Russian President, Mr Vladimir Putin.

The auction came after Yukos, which pumps more oil each day than Qatar, was unable to pay its back tax bill.

Last week, Yukos lawyers won a court injunction in the United States against the sale, reportedly frightening banks who had backed a multibillion dollar bid from Gazprom.

The injunction forbids Gazprom and foreign backers, believed to include Deutsche Bank and JP Morgan, from bidding for Yukos pending a hearing for Chapter 11 bankruptcy protection.

Although US law does not apply inside Russia, it can affect those parts of the bidding companies registered in the United States. Gazprom, which is the state gas monopoly, announced last night that it had no involvement in Baikal, but some observers questioned this.

"I see no plausible explanation for the theory that Baikal was representing competing interests," said Mr Paul Collison, a strategist with UBS in Moscow.

Yukos spokesman Mr Alexander Shadrin said its lawyers would pursue whoever was behind Baikal.

"We declare that the sale of Yugansk is illegal," he said.

"The victor of today's auction has bought itself a serious $9 billion headache."

This controversy comes at an awkward time for Russia, with Mr Putin mired in political disputes with the West over Chechnya and Ukraine.

Moscow's insistence that it follows the rules of the market economy, and is fit for early entry to the World Trade Organisation, has been badly hit over the Yukos affair.

Many were convinced that the prosecutions were political, not judicial.

Much will now depend on who turns out to be behind Baikal. The mystery is all the deeper because of the huge sums involved in the bid.

Few Russian firms have access to that sort of cash, and if the bid has been financed by an outside consortium, then it has been Western banking's best-kept secret.

Menatep, the bank through which Mr Khodorkovsky controls much of Yukos, announced yesterday that it will take its fight against bidding companies overseas, with Britain the first likely battleground.

"We intend to take legal action in multiple jurisdictions," said Menatep spokesman, Mr Sanford Saunders. "We anticipate the first filing to be in the United Kingdom."

Yukos has had a controversial history: Mr Khodorkovsky bought the oil holding for a knock-down price of less than $400 million in 1995, turning it into an oil giant and making himself Russia's richest man with an $8 billion fortune.

But he clashed with Mr Putin after financing a string of opposition parties in the run-up to parliamentary elections at the end of last year.

His prosecution for fraud, a charge that carries a 10-year maximum sentence, continues.