Russia's anti-monopoly service said Wednesday it was probing the British energy group BP's crisis-torn local joint venture TNK-BP on request from the state oil giant Rosneft.
The surprise announcement delivers another blow to a lucrative nine-year alliance that provides BP with more than a quarter of its oil output and billions of dollars in annual dividends.
BP last week said it was ready to sell its half in Russia's third-largest oil company after the venture's tycoon CEO Mikhail Fridman resigned and called for either the British or local partners to take full charge.
But it later emerged that TNK-BP's shareholder agreement prevented BP from disclosing important data about the joint venture to suitors that was required for any major divestment deal to go through.
Federal Anti-Monopoly Service deputy head Anatoly Golomolzin said he wanted to require firms to publish their shareholder agreements and was probing whether TNK-BP was keeping Western firms from entering the Russian market.
He revealed that the investigation was initially requested by Rosneft and had been ongoing for "some time".
"We are investigating the (TNK-BP) agreement on behalf of a request by Rosneft," Russian news agencies quoted Golomolzin as saying.
"We received a complaint saying that this agreement may be uncompetitive in nature."
Rosneft's new chief Igor Sechin -- a confidant of President Vladimir Putin who orchestrated a failed tie-up with BP when serving as Russia's energy policy chief -- has denied market rumours about making a bid for the British stake.