Russian state oil firm Rosneft has bought Yukos's core unit to effectively nationalise 11 per cent of the country's crude output and deal a heavy blow to Yukos's attempts to halt its own dismemberment.
A Rosneft official told reporters his firm had bought 100 per cent of Baikal Finance Group, the shock winner of Sunday's auction of core Yukos unit Yuganskneftegaz.
The Yugansk auction was the culmination of a Kremlin campaign to crush Yukos's politically ambitious principal owner, Mr Mikhail Khodorkovsky, and seize control of strategic sectors of the economy sold off in the chaotic privatisations of the 1990s.
The Rosneft purchase underscores the Kremlin's determination to prevail and claim Yukos's prize asset for itself.
It will make Rosneft one of Russia's biggest oil firms with production of 1.45 million barrels per day, almost as much as OPEC member Libya.
The move also confirms the ascendancy of hardliners in the Kremlin, known as siloviki, sworn to bringing to heel business "oligarchs", who rose to prominence in the post-Soviet chaos of the 1990s by acquiring state assets on the cheap.
President Vladimir Putin was due to give a Kremlin news conference at noon (local time) - already scheduled some days ago - and seemed certain to comment on the affair.
Rosneft is due to be merged with state gas monopoly Gazprom , which was itself the favourite to buy Yugansk at the auction but was barred by a Houston court considering Yukos's petition for bankruptcy under $27.5 billion in back-tax bills.
The auction was won by Baikal, a previously unknown firm that cast a winning bid of $9.4 billion.
Yukos lawyers have argued that Gazprom had illegally taken part in the weekend sale of Yugansk, even though it did not lodge a bid, and that it was therefore in contempt of the Houston court's restraining order.