Russian partners inside the TNK-BP joint venture are talking to financiers about raising money to make a potential $20bn (£12.4bn) cash offer for BP’s stake in the oil and gas business as early as next month.

The revelations have been met with a steely silence from BP which has developed a profound scepticism about the Moscow-based oligarchs who are still suing the British company in a Siberian court.

The Russian partners who control the Alpha Access Renova (AAR) consortium had previously been offering to purchase half of BP’s 50% stake in TNK-BP – but rival interest from state-owned Rosneft has forced it to think again.

“They [AAR] have decided they will make a cash offer for the full BP stake and are beginning to talk to banks,” said sources close to the Moscow-based consortium.

Asked why they had changed their minds, the source said: “Clearly circumstances have changed and we are dealing with a bidding situation with Rosneft offering cash and shares and BP making clear it is not interested in selling only half of its stake.”

The BP share in TNK-BP is generally thought to be worth $20bn-$25bn but AAR shareholders Mikhail Fridman, Viktor Vekselberg and Len Blavatnik, have apparently not yet decided exactly how much they will offer.

AAR had previously said it was willing to pay $10bn for a 25% stake in TNK-BP, Russia’s third-largest oil company. BP, which needs cash to help pay for potential liabilities from the Deepwater Horizon accident in the Gulf of Mexico in 2010, has already talked to Rosneft and Vladimir Putin about a cash and share swap worth $20bn.

BP, which has had a turbulent relationship with AAR, may see benefits from retaining a stronger foothold in Russia through a tie-up with Rosneft even in the face of a full cash offer from the oligarchs.

The company declined to comment but outside industry sources said BP senior officials were wary that any offer from AAR was likely to come with strings attached. The previous offer of $10bn from AAR was conditional on $3bn being subtracted to take account of the court case in Omsk being dropped by AAR.

Claims were made against BP in West Siberia on the grounds that the British company’s now-failed attempt to set up a rival joint venture with Rosneft had damaged the TNK-BP share price.

Under the TNK-BP shareholder agreement, only AAR could have an offer accepted before a 90-day deadline from the date when BP announced it wanted to sell its stake elapsed on 17 October. Other bidders may then step in with formal offers, creating the prospect of a bidding contest.

Bankers have told Reuters that it would be possible for AAR to finance a significant portion of any purchase by leveraging up TNK-BP, which has paid out $19bn in dividends since BP came into the venture in 2003. But other commentators are sceptical and argue that Kremlin politics could be the deciding factor in any asset sale.

There has also been suspicions that Fridman and the other oligarchs would ultimately like to sell their 50% stake in TNK-BP also and have hinted it would see Rosneft as the ultimate buyer.

Igor Sechin, chief executive of Rosneft and traditionally close to Putin, was quoted by Russian news agency, RIA Novosti, last week as saying his company would “consider” any offer of sale by AAR.

TNK-BP posted a record $14.6bn in profits last year and its gross debt was $8bn at the end of 2011, all of it unsecured and most of it long-term. That indicates, some bankers say, TNK-BP’s ability to sustain a higher debt burden to finance a possible buyout by AAR.

BP has already targeted the raising of $30bn to pay off any claims from the Macondo well blowout in which 11 people died and 17 others were injured. The Department of Justice is still planning to sue BP for gross negligence in a court case scheduled to start in January.