LONDON - The Russian state-owned oil company Rosneft is close to agreeing to buy a stake in TNK-BP, the country's third-largest oil company, from the British energy giant BP, a person with direct knowledge of the matter said on Thursday.
Under the terms of the proposed deal, Rosneft, which has close ties to Vladimir V. Putin of Russia, will offer a cash-and-stock deal on Thursday to acquire BP's 50 percent stake in TNK-BP, according to the person, who spoke on condition of anonymity because he was not authorized to speak publicly.
The deal could be worth up to $28 billion, though the person said the ongoing discussions might still not lead to a definitive deal.
BP's chief executive, Robert W. Dudley, is due to meet with his Rosneft counterpart, Igor Sechin, on Thursday, and the deal will be put to the British energy company's board on Friday, the person added.
Rosneft's move to acquire BP's stake in TNK-BP comes as the Russian energy company also is finalizing an agreement to buy the holdings of the four billionaires who own the remaining stakes in TNK-BP.
The two agreements could reshape the global oil sector by giving Rosneft control of a greater share of Russia's large energy reserves.
Mr. Putin has tried to reverse the privatizations of the country's energy industry since the fall of the Soviet Union, and Rosneft has benefited from close ties with the Russian president to become the country's largest oil company.
The proposed sale of its stake in TNK-BP follows months of discussions by BP as it looks to sell its ownsership in the profitable joint venture. For years, the British energy company has tried to extricate itself from the contentious business, which has been beset with infighting between BP and its Russian business partners.
Under a shareholder contract with BP, the Russian billionaires who own a stake in TNK-BP were not formally allow to agree to a sale of their holdings in the com pany until a grace period of obligatory good-faith negotiations with BP expired yesterday.
Arkady V. Dvorkovich, a Russian deputy prime minister, told the Russian Information Agency on Wednesday that discussions by Rosneft to buy the billionaires' stake were ongoing, but the government had yet to receive formal notice of Rosneft's plans.
‘‘There have not been any requests for approval so far, at least formal,'' Mr. Dvorkovich said.
In morning trading in London, shares in BP rose 1.1 percent.
