Russian gas monopoly Gazprom has approved its $13.1bn takeover of the fifth largest Russian oil firm Sibneft.
The deal will give the Kremlin control over one-third of Russian oil output.
Gazprom chairman Dmitry Medvedev said the deal would increase competitiveness at the world's largest gas producer, and a key supplier to Europe.
It also said the takeover would allow it to compete with global majors on the world's oil markets.
Gazprom had signed a binding agreement to buy 72.663% of Sibneft's shares from Millhouse Capital - an investment vehicle belonging to Mr Abramovich, who also owns Chelsea football club.
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Gazprom's Sibneft Acquisition Gives More Power To Kremlin
Russian gas monopoly Gazprom has approved its $13.1bn takeover of the fifth largest Russian oil firm