Russian investment consortium Alfa-Access-Renova will offer to buy half of BP plc's 50% stake in Russian oil producer TNK-BP Ltd. by mid-July, launching an offer ahead of a deadline written into the JV partner's shareholder agreement.

AAR, which is led by former TNK-BP CEO Mikhail Fridman, has until "the middle of the month" to make its bid, two sources with knowledge of the situation said on Friday, July 6.

London-based BP said on June 1 that it planned to sell its 50% stake in the nine-year old JV after it received "unsolicited indications of interest." Analysts have suggested the stake could be worth about $25 billion.

Under the terms of the TNK-BP shareholder agreement AAR has 45 days from the day BP said it intended sell its stake to officially declare its interest in buying the holding. That would mean that AAR would have to make its offer by about July 15. AAR will then have a further 90 days to negotiate a deal, during which time BP can talk to other interested parties.

"The timetable was triggered by BP's declaration and it is my understanding that an offer for half of BP's holding will be made in line with that timetable deadlines," said a source who asked not to be named as the shareholder agreement is confidential.

The consortium of Russian-born billionaires last month told BP's institutional shareholders that it was willing to either sell its 50% stake in TNK-BP to BP for cash and a significant minority equity stake or buy 25% of BP's holding.

Neither option is likely to appeal to BP, which has had a hugely fractious relationship with its JV partner. "We doubt whether either solution is massively palatable to BP in its current form," Citigroup Inc.'s London-based analysts Alastair Syme and Michael Alsford wrote on June 21. They noted that the sale of a 25% stake in TNK-BP would leave BP with a minority stake that it might struggle to sell at a later date.

"The shareholder agreement is confidential and we have no comment on it," said David Micholas, BP director, Group Media. "There have been reports of AAR's interest and while we haven't yet seen anything if they do express an interest we will consider it."

AAR believes that it is in a position of strength with regards to its offer as it suspects that BP does not have another bidder. The Russian partners have not been asked to allow a third party to see the shareholder agreement, a move that would almost surely form part of any due diligence process.

TNK-BP accounts for about 30% of BP's production and reserves and delivers to the UK oil giant roughly $3.8 billion in annual dividends, about 20% of BP's earnings.

The relationship between BP and AAR has broken down to the point where the board has not met since December, when several directors quit. TNK-BPs management crisis deepened at the end of May when Fridman quit as CEO. He has not since been replaced.

Shares in BP plc traded Friday at 428.3 pence ($6.67), down 5.9 pence or 1.36% on their previous close.