The Federal Security Service said that on March 12 officers detained TNK-BP's Ilya Zaslavsky, along with his brother Alexander, who has links with the British Council, for "illegally gathering secret commercial information for the benefit of several foreign oil and gas companies, in order to give them advantages over Russian competitors."
The British Council, the U.K. Embassy's cultural arm, said Ilya Zaslavsky was not on its staff but was an important partner as a member of the embassy's alumni club for Russian graduates of British universities.
TNK-BP's stock fell 4.5% on the RTS exchange after the news.
The FSB said charges were brought against the Zaslavsky brothers on March 18.
On Wednesday, police seized documents from the central office of the Russian-British venture TNK-BP, one of the largest crude producers in Russia. The Moscow office of British oil major BP was also raided by police.
Security officials said evidence had been found during the searches, including copies of official documents, papers on oil and gas production believed to contain a commercial secret, and ID cards belonging to foreign military organizations and to the CIA.
The British Council, whose regional offices in Russia were closed down earlier this year, voiced its concern over the arrests.
Britain has been involved in a number of espionage scandals with Russia before. Russian prosecutors launched a criminal case against Russian government officials for illegally transferring photocopies of confidential documents to a TNK-BP representative in October 2006.
In January 2006, Russia's security services also exposed four British spies who had a diplomatic shelter in Moscow and caught an advanced electronic device for spying. A Russian citizen was detained trying to establish radio contact with British services. The British Foreign Office then expressed concerns over the reports.