Russia’s Unified Energy System (UES) power giant has been slapped with a Rb 3.68 billion ($131.3 million) back-tax bill for 2001, a move which has heightened investor concern over state intervention in the energy sector.

UES is mounting a legal challenge to the bill, which followed a review of its accounts in 2004, but lost an appeal on the judgement in March. UES shares were pushed down on the news as Russian authorities continue to lash companies with back-tax bills. The news comes as oil giant Yukos faces a tax bill of more than $25 billion and its chief executive Mikhail Khodorkovsky is convicted on tax-evasion charges.

However, while the move has raised concerns, the bill is hardly significant when considered against the world’s largest energy company and is seen by some analysts as little more than a government measure to demonstrate equality within the tax system.