Police seized documents yesterday from the headquarters of TNK-BP, the Anglo-Russian oil giant, in a raid that raises new fears over Kremlin pressure on foreign investors.

Officers from the Interior Ministry searched the company's Moscow headquarters in what a police spokeswoman described as a criminal inquiry into Sidanko Oil, a former subsidiary. Sidanko was one of the companies merged to form TNK-BP in 2003.

Staff were told over the company intercom that investigators had entered the building.

Marina Dracheva, TNK-BP's spokeswoman, said: “We will always be prepared to co-operate with any law enforcement agencies if such a need arises.”

TNK-BP is Russia's third-largest oil company, jointly owned by BP and a group of Russian billionaires including Mikhail Fridman, of Alfa Group, the financial-industrial conglomerate, and Viktor Vekselberg, of Renova, the asset management company.

Shares in TNK-BP fell 10 per cent on the Russian stock exchange after news of the raid, Interfax reported.

Police conducted a search of the top floors of TNK-BP's offices in Moscow, initially preventing staff from entering or leaving.

Two individuals were said to be being interviewed by police. Neither BP nor its Russian affiliate would comment on the reason for the investigation, but observers noted that Sidanko was no longer an independent entity and that suggested the Russian authorities might be focusing on transactions relating to Sidanko before the TNK-BP merger. TNK-BP acquired a stake in Sidanko from Vladimir Potanin, the billionaire tycoon, for $480 million in 1997 but almost lost its investment in a tussle in the bankruptcy courts with Tyumen Oil, controlled by Mikhail Fridman and his partners, Viktor Vekselberg and Len Blavatnik.

Moscow oil analysts believe that the Kremlin wants BP's Russian partners to sell their half-interest in the joint venture to a state-controlled entity, such as Gazprom or Rosneft. A lock-in clause that formed part of the original joint venture has expired, leaving the partners free to dispose of their shareholding, but they have insisted that they wish to retain their stakes.

TNK-BP has faced growing state pressure as the Kremlin seeks to strengthen its grip on Russia's energy reserves. It has been the target of more than $1 billion (£497 million) in tax claims and was forced to cede control of the giant Kovykta gasfield in Siberia last year to the state-owned Gazprom. Shell was also squeezed out of the Sakhalin-2 oil and gas project last year in favour of Gazprom.

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