European Union competition regulators have opened a case against Russia's state-controlled Gazprom energy giant.
EU Competition Commissioner Margrethe Vestager said Gazprom's behaviour in central and eastern European member states, where it sometimes almost fully controls the gas market, could amount to an abuse of its dominant position.
Ms Vestager said she was investigating Gazprom's sales policies throughout most of the EU's eastern rim, from Estonia in the north to Bulgaria in the south.
The complaints are clustered around three issues: preventing cross-border flows of gas to other EU nations, charging unfairly high prices and demanding to keep control of the pipelines in return for gas.
The move comes at a time when the EU has already imposed economic and political sanctions on Russia for its involvement in the violence in eastern Ukraine.
Gazprom has 12 weeks to react to the EU allegations.
The decision to send an official statement of objections comes after some EU leaders and Moscow reached a peace deal that calmed the conflict in eastern Ukraine, where rebels have fought the Kiev government. The EU has long claimed President Vladimir Putin has been stoking the conflict for Russia's gain.
The standoff over Ukraine has forced the 28-nation EU into a sudden rethink of its energy policies to make it less reliant on Russia and Gazprom.
At an economic level, the 28-nation EU is Gazprom's most lucrative market. Gazprom's dominance in some nations has allowed it to play a huge role in international politics. By hindering gas trade among eight nations in the EU's eastern rim, it could enforce a "divide-and-rule" policy that gives it excessive power, the EU said.
All companies that operate in the European market - no matter if they are European or not - have to play by our EU rules, the EU said.