Russian state-controlled oil company Rosneft has signalled its interest in acquiring BP's share in its Russian joint venture TNK-BP.
BP is now being courted by two potential buyers.
TNK-BP's Russian shareholders recently confirmed they are eager to buy the British firm out too.
Rosneft said in a statement that it has informed BP of its interest in buying its stake in the firm.
The parties have agreed to start the negotiations.
TNK-BP, Russia's third-largest oil producer, is owned on a 50-50 basis by BP and AAR, a consortium of Russian billionaire shareholders.
TNK-BP has been mired in a lengthy dispute after AAR blocked BP's potentially-lucrative deal with Rosneft last year.
AAR has also indicated its interest in buying out BP. The British oil firm and the Russian tycoons have 90 days to negotiate in good faith, under the firm's shareholders' agreement.
A 50% stake in TNK-BP could be worth about $18 billion, according to the company's market valuation.
Rosneft said it believes that its purchase of BP's share in the Russian venture "would be in the best interest of both Rosneft's and BP's shareholders."
Although mired in boardroom disputes for much of its existence, TNK-BP has generated healthy returns both for the British oil giant and the Russians.
Over the past years, TNK-BP's output has accounted for more than a quarter of BP's total global production. The British oil company drew some $3.7 billion in dividends from the venture last year alone, receiving a total of $19 billion since the company's formation in 2003.
BP announced on June 1 its intention to sell some, or all, of its shares in TNK-BP.
TNK-BP's shares at Moscow's MICEX stock exchange were up 1.9%, outperforming the wider market. BP shares up 1.3% at 436.15 pence in early London trading.
