The Russian authorities have begun a fresh investigation into TNK-BP's tax arrangements, just hours after Gordon Brown held talks with Dmitry Medvedev, the Russian President, over the battle for control of the 50-50 joint venture company.

The new inquiry by tax and labour inspectors increases the pressure on BP, which will meet the joint venture's Russian shareholders on Friday when it is expected that the group will face renewed calls to dismiss Robert Dudley, TNK-BP's chief executive, who is backed by BP.

Tax inspectors have demanded reams of documents relating to payments made to BP specialists seconded to the company between 2006 and 2007. At the same time, Moscow's labour inspectorate yesterday launched a fresh investigation into whether the company’s main management unit and Mr Dudley are in compliance with labour laws, according to the Financial Times.

The investigations are part of a wave of regulatory pressure that has swept over TNK-BP since the dispute with the four Russian billionaires, who control 50 per cent of the company, began. The joint venture's Moscow headquarters were raided by the Federal Security Service in March after an employee was charged with industrial espionage. TNK-BP has also been the target of three labour inspections and a tax investigation.

Mr Dudley, the embattled chief executive, narrowly held on to his job on Monday after the billionaires failed in their attempt to fire him.

Mr Brown raised the problems at the £19.2 billion company on Monday during his first meeting with President Medvedev at the G8 summit in Hokkaido. Sergei Prikhodko, Mr Medvedev's foreign policy aide, said that the President had reiterated his view that this was a private dispute between shareholders.

The Russian shareholders want greater representation on TNK-BP's boards and fewer BP secondees at the company. They are also at loggerheads with BP over the company's strategy, arguing that the British side is blocking their demand for aggressive foreign expansion.

Separately, the Russian billionaires, who are fighting BP for control of the joint venture, are taking their case directly to some of the British company's shareholders with a series of meetings in London.

Stan Polovets, chief executive of AAR, the consortium through which the Russians hold their 50 per cent stake in TNK-BP, is understood to have met investors including Fidelity and BlackRock last month.

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