(BRUSSELS) - EU regulators will announce on Wednesday anti-trust charges against Russia energy giant Gazprom, sources close to the matter told AFP, in a move that risks inflaming tensions with the Kremlin.

The European Commission formally opened its probe in September 2012 and has significantly delayed moving forward due to the crisis with Moscow over Ukraine.

The EU will officially complain that Gazprom was hindering competition in Central and Eastern European gas markets, where the company benefits from a dominant position.

If found responsible, Gazprom risks fines as high as 10 percent of the company's overall sales, which amounted to the ruble equivalent of 93 billion euros ($100 billion) in 2013, the latest data available.

The countries involved are Lithuania, Estonia, Bulgaria, Czech Republic, Hungary, Latvia, Slovakia and Poland.

"What we're looking at is whether customers are getting the best prices or if a dominant company is using its remarkable strength to get different prices for different customers," EU Competition Commissioner Margrethe Vestager told Bloomberg TV without confirming the probe.

"It means quite a lot for a country if it pays more for its gas than maybe it ought to do," she said.

The complaint against Gazprom will land a week after Vestager formally charged search engine Google with abusing its dominant position, in another high-profile case for the commission, the EU's executive arm.

Gazprom's leading position and close ties to the Russian government has made the handling of the case especially sensitive for EU regulators.

Russia alone supplies about a third of the EU's gas requirements -- it bought 125 billion cubic metres from Gazprom last year, with half that amount going through pipelines that cross Ukraine.

But repeated disputes with Ukraine have led Gazprom to suspend gas supplies on several occasions and made Europe's energy supply a crucial aspect of conflict between the Western-backed government in Kiev and pro-Russian rebels in eastern Ukraine.

Faced with the worst tensions with the Kremlin since the Cold War, the EU backed off from pushing the Gazprom case while also desperately seeking to diversify its energy supply.

Gazprom chief Alexei Miller is threatening to put an end to the supply of gas through Ukraine in 2019, ratcheting up the tensions further.

News of the probe comes a day before Miller visits Athens amid conflicting reports that Russia and Greece will sign a pipeline deal in which Gazprom would pay billions in an advance payment to the cash-strapped Greek government.

The radical leftist government in Athens is currently caught in a bitter row with its European partners over its massive debt obligations.

Greece has hinted it could turn to Moscow for help without a resolution to the feud.