European competition commissioner Margrethe Vestager will accuse Gazprom of overcharging buyers in eastern Europe, 'EU sources' told Reuters.
People familiar with the case told the Financial Times that Vestager may focus the Gazprom investigation in a similar way.
The European Commission is responsible for investigating competition-restricting agreements and abuse of dominant market position under the Treaty on the Functioning of the European Union (TFEU).
Vestager said last week that an 'energy union' was a priority for the Commission, and that it would take action against energy suppliers which stood in the way of the free flow of energy between EU countries.
"Integrating energy markets is a key objective of the Energy Union," she said in a speech in Washington last week. "If gas and electricity flowed freely around the EU, we would make a big step towards economically sustainable, environmentally friendly, and socially inclusive solutions."
"Consistent enforcement of the rules helps give investors the legal certainty and predictability they need," said Vestager. "Coordination can be improved by applying the rules in the EU Treaty on financial support granted by EU countries. And acting decisively against energy companies that harm rivals, block energy flows from one EU country to another, or threaten to close the tap can help deter others."
The Commission has been looking at Gazprom for over two years now, and Vestager told the Wall Street Journal in February that she did not see any political dimension to the case.
In response to suggestions that the timing was bad for an investigation into a Russian company, she said: "It’s very important for me to make sure that any company in the European market is being faced with the same set of rules and the same effort of enforcement [ … ] I think any neighbouring company in the gas market would find it very strange if we had a case and we did not pursue it."