The European Commission is looking into allegations that Gazprom abused its dominant position in the Bulgarian wholesale gas market, the Russian gas producer said in a prospectus for a bond issue.
The Bulgarian case increases the pressure on the Kremlin's energy champion within the European Union, where it has already come under fire in eight countries for allegedly abusing its dominant position and overcharging customers.
"The European Commission sent to Gazprom and Gazprom Germania an official request for information in connection with an alleged violation of EU antitrust legislation related to natural gas supplies to Bulgaria," Gazprom said.
"We have always supported competition in the gas market and have sought to comply with all international, European and national laws and regulations in countries where we operate," it said in its March 11 prospectus for a eurobond.
Gazprom supplies roughly a third of the EU's gas and it has been accused in an ongoing EU case of abusing its dominant position and overcharging customers by up to 40 percent in Poland, Hungary and six other countries.
An EU source said the Commission was looking separately into the alleged interruption of gas deliveries from Gazprom to Bulgaria's Overgas.
In January, Bulgaria's state energy regulator launched emergency checks into Overgas after Gazprom suddenly halted supplies to the private company on Jan. 1.
Overgas, which is 50 percent controlled by Gazprom and its export unit, supplies about 10 percent of Bulgaria's gas.
Gazprom and its exporting arm, Gazprom Export, have agreed to exit Overgas by the end of April and an Overgas spokeswoman has said problems around deliveries at the start of the year were probably linked to the pending shareholder changes.
"The European Commission is closely following the competitive conditions as regards the Bulgarian gas market," the Commission spokesman, Ricardo Cardoso, told Reuters.
(Reporting by Oksana Kobzeva and Vladimir Soldatkin in Moscow, Foo Yun Chee and Barbara Lewis in Brussels)