The advocate general at the EU Court of Justice (ECJ) said that "the infringements of EU law of which Slovenia accuses Croatia are ancillary to the issue of determining the boundary between those two states, which is a matter of public international law".

He noted that the situations in which the EU is bound by international law are well-established, and "international conventions that do not fall within those categories are not acts of the EU and do not bind it".

In bringing its case to the court, Slovenia argued that Croatia infringes several articles of EU law by refusing to implement the award as declared by the mutually appointed arbitration tribunal in June 2017.

While the opinion is not binding, a point emphasised by the ECJ in releasing the reasoning of the advocate general, it has raised eyebrows in Slovenia, which has been counting on courts to force Croatia to comply with the border arbitration award.

Foreign Minister Miro Cerar said the advocate general's reasoning is weak and had "absolutely no effect on the validity of the arbitration award".

Cerar said that the ECJ had so far decided in only five cases brought against a fellow member state and that the court followed the advocate's position in only three of the cases.

Similar opinions came from the top echelons of Slovenian politics.

President Borut Pahor said that the opinion did not change the fact that the border between Slovenia and Croatia had been drawn by a Hague-based international arbitration tribunal.

And Prime Minister Marjan Šarec said that Slovenia remained determined to implement the arbitration decision and that bilateral talks or returning to square one was out of the question.

Legal experts likewise think the opinion does not change much, but they have indicated Slovenia may have to look beyond the courts.

Verica Trstenjak, a former advocate general at the ECJ, said the arbitration award will still be valid regardless of the lawsuit's admissibility.

"If Slovenia loses the suit, it does not mean the country has lost anything because the situation will be the same as it was the day before it brought the suit. Arbitration is still valid."

But she said that in the long run a diplomatic solution will have to be sought. According to her, the issue will need to be resolved by way of diplomacy, taking into consideration the existence of the arbitration award.

Janja Hojnik, a professor of EU law at the Maribor Faculty of Law, highlighted the absence of case law and Slovenia's weak position vis-a-vis Croatia.

"We know that Slovenia's legal options are limited. It doesn't have the option of direct enforcement, a fine, like it would have under EU law. Slovenia is in fact in a worse position under international law and Croatia knows that, which is why it has reacted the way it has, by not complying with the arbitration award."

Hojnik does not think the advocate general's opinion augurs well. "The EU in general, not just the EU court but also the European Commission, does not fully understand the political situation in this region or the difficulties related to Croatia's unresolved border issues."

The court's judges will now start deliberations on the admissibility of the case. "Judgement will be given at a later date," the ECJ's release said. Unofficially, it is expected in the first quarter of 2020.