Erjavec said there were two options: either the EU Commission sues a member state, or a member state notifies the Commission it has lodged a lawsuit, whereupon the Commission examines whether EU law has been violated.

The Commission first attempts mediation, but if that is unsuccessful it may pursue the lawsuit itself or leave the injured member state to proceed in front of the EU Court.

This procedure takes three to four months "which is why we assume the lawsuit could be filed in spring," Erjavec said after a working meeting on arbitration hosted by Prime Minister Miro Cerar.

Come 29 December, Slovenia will "shift to the phase of actual implementation" of the arbitration tribunal's award and every violation will be considered an infringement of EU and international law.

"These are legal facts that will be relevant in particular in the use of legal means," according to Erjavec, who said implementation was not a unilateral act but constituted observance of international law.

"As an EU and NATO member, and aspiring OECD member, Croatia will simply have to respect the arbitration award, which is part of international and EU law."

Commenting on warnings by Croatian Prime Minister Andrej Plenković that no decision could be applied unilaterally and that Croatia would defend its territory, Erjavec said he expected "Plenković to refrain from any incidents."

"The arbitration tribunal's award is very clear about what is Croatian and what is Slovenian," he said. "Any conduct bypassing the arbitration award and international law shall be considered a violation."