Roman Abramovich bought Chelsea FC in July

Parliamentary deputy Vladimir Yudin on Tuesday unveiled a letter he wrote to state prosecutors last month urging them to probe the purchase of certain oil company stocks by Sibneft in 1995.

Mr Yudin also suggested that the sale of some state-owned Sibneft shares in 1996 and 1997 was illegal.

He claimed that Sibneft had defrauded the Russian government out of about $6bn (�3.5bn), and called on the authorities to freeze the company's shares.

Mr Yudin, who is believed to have close ties to the Kremlin, pushed state prosecutors into launching their investigation of oil giant Yukos, currently in the final stages of a merger with Sibneft.

Last week, the Yukos probe led to the shock arrest of the firm's chief executive, Mikhail Khodorkovsky, a leading member of Russia's super-rich business elite.

Investor jitters

Russian prosecutors also froze more than 40% of Yukos' shares.

Mr Khodorkovsky, who resigned as Yukos chief executive on Monday, is being held in a Moscow jail awaiting trial on fraud and tax evasion charges.

The episode has stirred fears that Kremlin hardliners are leading a crackdown on Russian free enterprise, unnerving foreign investors and triggering a sharp slump on the country's stock exchange.

It is widely believed in Russia that Mr Khodorkovsky's arrest is a politically-motivated response to his support for liberal opposition groups.

Analysts says this broke a tacit agreement to stay out of politics in return for avoiding investigation of his financial affairs.

Mr Abramovich, who like Mr Khodorkovsky made his fortune through rushed privatisation deals in the 1990s, is best known in the UK for his surprise acquisition of Chelsea football club this summer.

He is reported to have sold off many of his Russian business interests in recent months, fuelling speculation that he is planning to withdraw from the country altogether.