Geoenergo, which is exploring the gas field in conjunction with the British firm Ascent Resources, said it had requested the permit from the Environment Agency as a precaution.

Hydraulic fracturing, known as fracking, had been conducted at both wells, known as Pg-10 and Pg-11A, in 2011.

"Considering that the wells had been dormant for six years between their construction and the start of production, we knew it was likely that renewed hydraulic stimulation would be needed," the company said.

Since procedures for the acquisition of permits are long, Geoenergo decided last year to submit the paperwork.

Geoenergo had submitted the application in May 2017 and amended it at several stages, most recently this March, even as it claimed in November 2017 it did not yet know when renewed fracking would be needed.

The company would not directly comment on that.

The application shows Geoenergo would use 10,000 m3 of water to stimulate the wells at a depth of 2,700 and 3,500 metres, respectively.

Gas has been pumped at Petišovci since November 2017. It is sold unpurified to Croatian refiner Ina. While daily production numbers are not disclosed, media reports suggest daily output is about 60,000 m3.

Original estimates put the potential of the gas field at about 12 billion m3 of gas, with Ascent originally projecting a daily output of 300,000 m3.

Gas exploration at Petišovci has long been the subject of criticism from local green groups, but the initial uproar over fracking opposition subsided several years ago after Geoenergo and Ascent assuaged fears about fracking polluting the groundwater.

Mihael Kasaš, who has spearheaded a local civil initiative that opposes fracking, told the STA that the group would "resist" the planned additional fracking, but only if it receives support from municipal council in Lendava.

He has sent a message to all councillors but has so far received a statement of support from only one.

The fracking of Pg-10 and Pg-11A is an attempt to redevelop the Petišovci gas field, which has been producing small amounts of oil and gas since the Second World War.

Geoenergo has exclusive exploitation rights for a 97 km2 area covered in the Mura Depression which is bordered by Hungary in the north and the Mura River and Croatia in the south.