BP said on Friday it was considering a sale of its half-share of Russia's third-largest oil producer TNK-BP, possibly heralding the end of one of the UK oil major's most lucrative, audacious and troublesome ventures.

A sale could raise around $30 billion for BP, which would help to fund the ongoing cost of cleaning up the 2010 Gulf of Mexico oil spill, and the potential fines related to this.

Yet it would also cost BP annual dividends, which hit $3.7 billion last year, and around 30% of its oil and gas production.

BP had previously said TNK-BP was a core asset. The proposed sale is an admission that after years of squabbling with its partners, Alfa Access Renova (AAR), a group of four Soviet-born billionaires, the status quo was no longer tenable.

"We've worked hard to come up with a solution but haven't been able to do so," a source close to the situation said.

BP said in a statement on Friday that it had received "unsolicited approaches" and had notified AAR of its intention to pursue a possible sale.

The London-based group declined to say which parties had made the approaches. Given the Kremlin's desire to exert influence over the oil sector, suspicion is likely to fall on state-backed players such as Rosneft and Gazprom.

On Monday, one of the billionaires, Mikhail Fridman, resigned as chief executive of TNK-BP , citing a breakdown in relations with BP.

BP sources said they believed the move was a tactic to force BP to buy out AAR's stake in the venture.

Lucrative headache

BP invested almost $8 billion in cash, shares and assets to form TNK-BP with AAR in 2003 to expand into Russia, one of the few major resource-holders where foreign oil companies could invest.

It has been a huge financial success for BP, yielding $19 billion in dividends - typically around 10% of profits - in spite of high Russian taxes on oil production.

However, it has been a bumpy ride, prompting investors to attach a significant discount to the stake.

Shortly after the venture was formed, Russia became more hostile to foreign and private-sector investment. The Kremlin's preference to reserve larger fields for state-controlled groups has limited TNK-BP's ability to expand in Russia.

Meanwhile, BP has been locked in periodic disputes with its co-owners that have led to worries that BP could lose its interest with little or no compensation.

In 2008, BP Chief Executive Bob Dudley, then CEO of TNK-BP, was forced to flee Russia under what he described as a campaign of harassment from AAR.

Before that, BP had enjoyed effective control of TNK-BP but afterwards, Fridman became the main player.

BP offered to buy AAR out for around $32 billion in 2011, BP sources said at the time, following a flare up in the relationship prompted by BP's decision to sign an Arctic exploration deal with Rosneft.

That deal fell apart and AAR blocked the Rosneft tie-up, making use of a TNK-BP shareholder agreement which forces BP to use TNK-BP as its main vehicle for expansion in Russia.

A sale of BP's TNK-BP stake would free BP up to be able to sign such ventures in future, potentially offering BP the potential to ditch a troublesome, low growth asset for higher growth opportunities.

This strategy of churning assets is one that Dudley has espoused to investors since being appointed in the wake of the oil spill. AAR was not immediately available for comment.