The European Union launched a . legal attack on Gazprom on Wednesday, stoking tensions . with Moscow as it accused the Russian gas giant of overcharging . buyers in Eastern Europe and hindering competition. European Commission antitrust chief Margrethe Vestager said . Gazprom was barring EU clients from selling on its gas to other . states - a particular concern in recent efforts to aid Ukraine - and pressuring governments to back its pipeline interests. Russian Foreign Minister Sergei Lavrov said that the charges were 'absolutely unacceptable'. The European Union launched a legal attack on Gazprom on Wednesday, stoking tensions with Moscow as it accused the Russian gas giant of overcharging buyers in Eastern Europe and hindering competition . European Commission antitrust chief Margrethe Vestager (pictured) said Gazprom was barring EU clients from selling on its gas to otherstates . State-controlled Gazprom is a vital supplier of energy to . Europe despite the EU's frequent political disputes with Moscow. The Commission's investigation, opened in September 2012, . had initially covered Poland, the Czech Republic, Slovakia, . Hungary, Bulgaria, Estonia, Latvia and Lithuania. Relations with Russia, and Gazprom in particular, have since . been poisoned more by the East-West confrontation over Ukraine. 'Gazprom is dominant in all these markets,' EU Competition . Commissioner Vestager told a news conference. 'Our preliminary . view alleges that Gazprom is abusing this position.' 'Gazprom has been able to charge higher prices in some . countries without fearing that ... gas would flow in from where . prices were lower,' she said of contracts with the three . ex-Soviet Baltic states and formerly communist Poland and . Bulgaria. Vestager said prices in some countries were as much as 40 . per cent higher than in others. Lithuanian President Dalia Grybauskaite welcomed the . charges, saying: 'The era of Kremlin-backed political and . economic blackmail draws to a close.' Russian Foreign Minister Sergei Lavrov said that the charges were 'absolutely unacceptable' Noting the investigation had begun before Russia annexed . Crimea from Ukraine, Vestager said 'this case is not political' but acknowledged some would see political elements in it. Gazprom responded by calling the charges, to which it has 12 . weeks to respond, 'unfounded' and saying it expected a . resolution within the framework of previous undertakings between . the Kremlin and Brussels 'on the intergovernmental level'. Russian Foreign Minister Sergei Lavrov said on Wednesday that the EU charges against Gazprom were an unacceptable attempt to retroactively apply the bloc's latest energy rules to earlier contracts. Asked to comment on the case, Lavrov said in an interview with three radio stations: 'All contracts in effect now that Gazprom signed with its partners, were signed with full respect of the legal regime that existed in the EU at the time. 'After the EU adopted the so-called third energy package... there have been attempts, and they continue now, to retrospectively, retroactively apply those requirements to old contracts as well. That is absolutely unacceptable.' The EU has brokered deals to keep gas flowing to Ukraine . despite the conflict between Kiev and Moscow, but efforts to . supply gas via eastern EU states have been hindered by contracts . with Gazprom that prohibit the re-export of Russian gas. Gazprom had also pressured Poland and Bulgaria into . accepting unfavourable terms over gas pipeline control and . construction, the commissioner said. The decision to move against Gazprom comes a week after . Vestager charged U.S. tech giant Google with abusing . its market power, following five years of hesitation by her . predecessor. She said, however, that in other cases she would be . willing to negotiate with firms without pressing charges. The Dane has appeared determined to challenge big corporate . powers since taking on the powerful post in November, regardless . of past offers of compromise from both Google and Gazprom. Despite insisting she would look only at the legal merits of . a case that focuses on Gazprom's pricing policies for different . customers, the accusations will not ease frictions with Moscow . over Ukraine in which gas supplies have played a major role. Gazprom offered concessions to Vestager's predecessor last . year in a bid to settle the case and avoid a possible fine but . talks failed over its refusal to cut prices in eastern Europe. The Russian behemoth, with annual sales of some $100 . billion, supplies about 30 per cent of the 28-nation EU's natural . gas. It has been under investigation since September 2012, . including for hampering the flow of gas across Europe. Vestager can order companies to change their business . practices and is theoretically able to levy fines of up to 10 . per cent of their annual global turnover.
Antitrust chief  said Gazprom is barring EU clients from selling on its gas . Russian Foreign Minister Sergei Lavrov said charges were 'unacceptable' 'Era of Kremlin-backed  economic blackmail draws to a close' - Lithuania .