Roman Abramovich, the multi-billionaire owner of Premiership football champions Chelsea, looks set to add another £6 billion to his cash fortune, according to newspaper reports in Moscow today, which suggest that Gazprom, Russia's state-owned gas monopoly, is to buy up shares in his oil firm Sibneft.

Sibneft’s core shareholders, led by Mr Abramovich, control 72 per cent of the company and are in talks to sell it to Gazprom for reportedly more than $10 billion.

Gazprom has told Sibneft’s owners they should regain control of a 20 per cent stake held by stricken oil company Yukos before the takeover. The remaining 8 per cent is freely floated.

State oil company Rosneft has said it expects to be awarded the 20 per cent stake in a court battle with Yukos . If Rosneft gets the stake and then increases it to 25 per cent, it would be able to block key decisions over how Sibneft is run.

Vedomosti, the Moscow business daily, quoted two sources familiar with the Gazprom plans as saying that the gas giant had managed to accumulate 3 per cent of Sibneft in the open market by the end of Friday, effectively signalling Gazprom had succeeded in keeping Rosneft at bay.

Some analysts said that if it was true that Gazprom had bought the 3 per cent, valued at $550 million on Friday, it was still likely to chase the 20 per cent stake to stop Rosneft getting hold of it.

"Even as a shareholder without a blocking stake, Rosneft does not appear to be welcomed by Gazprom," Adam Landes, oil and gas analyst at Renaissance Capital, said in a research note.

Analysts at Troika Dialog brokerage said buying shares in the open market was a cheaper and quicker method of getting the crucial 75 per cent plus one share, and had the additional advantage of leaving Rosneft with an orphaned holding.

"A 20 per cent stake would be left for Rosneft, which would be forced to sell it without the premium that it could demand if it had a blocking minority," Troika said.

Analysts said the news was bad for Sibneft’s minority shareholders, both because they are expected to lose out in a Gazprom takeover and because if Gazprom has got its 3 per cent there would be less demand for their shares.

Sibneft stock rose 19 per cent last week, its biggest weekly gain in four years.

Despite the wide expectation of the deal, one analyst sounded a note of caution. "Gazprom’s efforts to acquire Sibneft could still be thwarted either by a late intervention by Rosneft ... or if the banks balk at financing an asset that is subject to a serious legal threat," Anisa Nagaria, at Alfa Bank, told Reuters.