A mystery bidder on Sunday paid $9.35bn for the chief production asset of the Yukos oil company, winning an auction that confirmed Russia's reputation as one of the world's most opaque business environments.

A previously unknown company called Baikal Finance Group became the official owner of one of the country's largest oil production companies, which accounted for 60 per cent of all Yukos output. It was unclear who was behind Baikal or its finances.