Yukos, the troubled Russian oil giant, has been accused by Russian authorities of owing some $5 billion (€4.17 billion) in back taxes.

Russian news agency Interfax said the tax ministry had made the allegation in a letter to the public prosecutor's office.

Yukos, which is struggling to save plans for a merger with rival Sibneft, immediately denied that it had been evading tax payments.

"We consider the claim of 150 billion roubles ($5.05 billion) of tax arrears absolutely improper," said Yukos spokesman Mr Alexander Shadrin.

Yukos, whose former chief executive Mr Mikhail Khodorkovsky was arrested in October, has been subjected to relentless pressure from prosecutors in recent weeks.

The prosecutors, who have accused billionaire Mr Khodorkovsky of tax evasion and fraud, are widely believed to have acted at the bidding of the Kremlin, which is anxious to curb the powerful businessman's political ambitions.

Neither the tax authorities nor the prosecutor's office would comment on the Interfax report. Yukos has repeatedly denied it has cut corners when it came to paying tax.

Yesterday, economy minister, Mr German Gref, told Reuters that Russian companies must stop exploiting tax "loopholes", but said he was not singling out Yukos.

Analysts said Yukos's tax woes may have been a reason why Sibneft, which agreed in April to merge with it to create the world's fourth-biggest oil company, announced abruptly on Friday that it was suspending the deal.

Shareholders in Yukos say they are fiercely resisting calls from Sibneft's main shareholders, led by Mr Roman Abramovich, the owner of London soccer club Chelsea, to surrender management control of the merged company as the price of saving the deal.