Russia has launched a criminal probe into alleged breaches of licences at Rospan - a unit of TNK-BP, the oil and gas firm 50%-owned by BP.
TNK-BP contributes considerably to BP's output
It follows a move to revoke two of Rospan's gas licences in Siberia due to alleged environmental violations.
Some analysts have said Russia wants to reduce the influence of outside investors to increase the Kremlin's control over the energy sector.
Projects led by other giants including Shell and Exxon Mobil also face probes.
The two sites where prosecutors say there have been environmental breaches by Rospan are close to West Siberian sites operated by Gazprom.
An unnamed Rospan executive is at the heart of the investigation.
Rospan, which makes up 7% of TNK-BP, was initially an independent firm but was bought by former oil giant Yukos.
Yukos then sold Rospan to TNK-BP as Yukos tried - and ultimately failed - to avoid bankruptcy after it was hit with a huge tax bill.
The investigation into the TNK-BP unit is the latest in a number of moves by Russian authorities to scrutinise energy projects which involve foreign energy firms.
Last week, Russia's environmental agency asked for an extra month to probe the Sakhalin-2 oil and gas project - headed by Royal Dutch Shell - and warned it may prosecute its operator.
Natural resources minister Yuri Trutnev said that environmental breaches at that project broke five criminal codes.