MOSCOW, Sept 26 (Reuters) - The billionaire quartet that owns half of Anglo-Russian oil firm TNK-BP is interested in buying out BP's 50 percent stake, and will make a binding all-cash offer by mid-October, a source familiar with the matter said on Wednesday.
"We will pay cash, but have not yet determined the price," the source, who is close to the AAR consortium that represents shareholders Mikhail Fridman, German Khan, Viktor Vekselberg and Len Blavatnik.
AAR had said it was willing to pay $10 billion for a 25 percent stake in TNK-BP, Russia's third-largest oil company. BP, which put its stake up for sale in June, was not interested in that offer, sources said.
AAR expanded its bid after state oil major Rosneft said it wanted to buy BP's stake in TNK-BP for cash and stock in a deal that could be worth more than $20 billion. Such a deal would require government approval and is opposed by some senior Russian officials.