BRUSSELS, June 23 (Reuters) - EU antitrust regulators on Thursday opened a full-scale investigation into Hungarian oil and gas group MOL's (MOLB.BU) proposed acquisition of Austrian rival OMV's (OMVV.VI) Slovenian operations, concerned that the deal could hurt competition.

MOL and Croatian oil and gas company INA (INAI.ZA) , the biggest shareholder of which is MOL, announced the deal a year ago to acquire OMV Slovenija, Slovenia's second-largest petrol retailer. read more

MOL requested EU antitrust approval for the deal last month and did not offer any concessions to address the EU competition enforcer's concerns after a preliminary review.

The European Commission said it was concerned that the deal to merge the second and third-largest fuel suppliers in Slovenia could reduce competition.

"To ensure that fuel prices at the pump are not artificially inflated by lack of competition, we need to keep a close eye on consolidation," Commission Vice President Margrethe Vestager said in a statement.

The Commission also cited high barriers for potential rivals to enter the market. It will decide by Oct. 28 whether to clear or block the deal.

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