Life has become very uncomfortable for top TNK-BP managers
Senior foreign executives at BP's Russian joint venture could be forced to leave the country after their requests for work permits were refused.
Russian authorities have turned down more than half of the requests made by expat managers of TNK-BP as the battle for control of the firm escalates.
Russian investors have demanded British directors be sacked amid claims they are seeking sole Russian ownership.
BP described the latest blow as "surprising and disappointing".
Senior foreign executives, including chief executive Robert Dudley, will have to leave Russia by the end of the month unless they can secure new visas.
The existing visas of foreign staff expire then and, following the refusal by the Moscow authorities to grant extended work permits, they will need special permission to remain.
Staff are expected to appeal against the ruling on Tuesday, although there is little sign, at the moment, that the authorities will reverse the decision.
BP said the permit requests were properly "authorised" and the company had not encountered any problems in the past with similar requests.
"We are very disappointed to see that many of the expatriate staff working in TNK-BP will have to leave Russia and may not be able to return," it said in a statement.
"The loss of these staff will definitely damage TNK-BP, its performance and, by extension, the performance of the Russian oil sector."
There are only about 100 expatriate staff among the firm's 66,000 strong workforce - less than half of whom work for BP - but these include Mr Dudley and the chief operating officer Timothy Summers.
BP has accused certain Russian shareholders, who control 50% of the company, of trying to destabilise the business.
Critics believe the Russian authorities would like to bring TNK's lucrative oil operations under state control by selling the company to either Gazprom and Rosneft.
Russian officials deny this, saying they want to co-operate with BP, but accuse their foreign partner of treating the business as its subsidiary rather than a mutual venture.
Relations between the British and Russian governments are decidedly frosty with London accusing Moscow of refusing to co-operate with the police investigation into the poisoning of former KGB agent Alexander Litvinenko in London in 2006.