The European Commission has raided the offices of natural gas companies in Central and Eastern Europe in a bid to determine whether firms are engaging in anti-competitive practises.

The unannounced raids were in pursuit of an investigation by the Commission that is thought to focus on Russian firm Gazprom and its long-term supply contracts. The Commission said that the companies it raided “may have engaged in anticompetitive practices in breach of EU antitrust rules” or are “in possession of information relating to such practices”.

Gazprom says that it is co-operating with the investigation, which analysts believe illustrates growing concern in Brussels over Europe’s reliance on Russian natural gas.

Gazprom has received complaints from energy firms in Germany, Turkey and the Ukraine about the price of natural gas.

The European Commission suspects that in the upstream gas supply sector, competition may have been hampered or delayed by exclusionary behaviour, such as market partitioning, or exploitative behaviour, such as excessive pricing.

A statement from the Commission said: “The Commission is determined that consumers throughout the EU should enjoy the benefits of an integrated and competitive single European energy market that increases security of supply of energy at affordable prices. This means in particular ensuring the diversification of sources of supply and the free flow of gas once it has entered the EU.”

Gazprom is Russia’s biggest company and the world’s largest gas producer. It supplies around one-quarter of Europe’s gas needs.

According to Bloomberg, other companies whose offices were raided include RWE, E.ON’s Ruhrgas and Hungary units, OMV AG and Poland’s PGN.