The agency granted the investor the permit for a planned gas processing plant but demanded a separate environmental impact assessment to determine whether the UK company can step up extraction via hydraulic fracturing, which is crucial for the refinery that would be allowed to process 280,000 cubic metres of natural gas and a tonne of oil per day.

The March decision of the Environment Agency came after the original permit for the refinery, issued in 2015, had been successfully challenged by environmentalists.

The ministry agrees with the agency that an environmental impact assessment and a separate environmental permit were necessary because the location was close to water sources and because underground waters and agricultural land in the area do not have very good ability to regenerate.

Operating in a joint venture with Geoenergo, which is co-owned by the Slovenian state-controlled energy companies Petrol and Nafta Lendava, the UK company wants to extract gas on a large scale in Petišovci.

The separate permit procedure could further delay the implementation of the project in which more than EUR 50 million has allegedly been invested so far.

The UK company holds 75% interest in the project, Geoenergo's concession for the Petišovce gas however expires in 2022.

Hydrocarbon extraction in Petišovci started in 1943 and boomed in the 1980s. But after the oil refinery there was closed, the activity slowly died down.

Last December, Ascent Resources stepped up pressure on Slovenia to issue the environmental permit for its project by threatening to sue the government for damages.

This was after the then Environment Minister Jure Leben ordered an internal inquiry at the Environment Agency to see "whether inappropriate pressure has been exerted on employees" in relation to two Petišovci-related procedures.

The inquiry showed that there had been pressure and threats in both procedures and that the autonomy and independence of the decision-making authority had been violated. The findings prompted Joško Knez to resign as the agency's director.