An international arbitration court will deliver a ruling on Thursday (29 June) that is likely to reignite a border dispute between Slovenia and Croatia, putting EU institutions in an uneasy position.

The court, which is based in The Hague, Netherlands, will give its verdict on where exactly the 670-kilometre long border between the two countries runs. It also has to define a junction between Slovenia's territorial waters and the high seas in the Adriatic, in the Gulf of Piran.

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The arbitration process was agreed by the two countries in an EU-brokered agreement that lifted Slovenia's veto on Croatia's EU accession in 2009.

Slovenia is an EU country since 2004. Croatia joined in 2013 with respect for the ruling stipulated as a condition in its accession treaty.

But while Slovenia has said that it would respect the ruling - called an award - Croatia has warned that it no longer recognises the authority of the court.

"Croatia is neither going to accept nor reject the arbitration's ruling for one very simple reason: the tribunal doesn't exist," Croatian president Kolinda Grabar Kitarovic said recently.

Croatia has contested the arbitration procedure and the court's competence since 2015, after it was revealed that a Slovenian official had contacts with the Slovenian member of the arbitration tribunal.

When the five-strong tribunal in charge of the border case was established, chaired by a French judge, it included a Slovenian and a Croatian member.

The Slovenian official was dismissed by her government, which said she had acted on her own. The judge resigned, soon followed by his appointed successor.

The tribunal was then reshaped, with a Norwegian and a Swiss judge replacing both the Slovenian and the Croatian members.

The Gulf of Piran

The arbitration court said last year that "Slovenia has violated provisions of the arbitration agreement", but that the "arbitration proceeding shall continue".

Kitarovic said however that Croatia was "ready to start the negotiations immediately" after the arbitration court's ruling, and she invited Slovenia "to the negotiating table".

"I believe that we must show the maturity and ability to resolve this issue on a bilateral basis," she added.

Slovenian prime minister Miro Cerar, for his part, admitted that the arbitration decision will need to be discussed with the Croatian side," but he ruled out "negotiating again" about the border.

The dispute, which dates back to 1991, when Slovenia and Croatia became independent, concerns a few small stretches of land - the biggest one is 113 hectares - in which 100 people live. The court's ruling is expected to imply some changes in the flows of rivers, to the benefit and detriment of both sides.

The most contentious part of the dispute, which was also at the centre of Slovenia's veto on Croatia's EU accession until the arbitration agreement, is about the Gulf of Piran.

With several ports in Italy, Slovenia and Croatia, the gulf is a high-traffic maritime zone, as well as an important fishing area. Slovenia claims a junction between its territorial waters and the high seas - a corridor crossing Croatia's waters - as a matter of sovereignty, with consequences over which country authorises navigation in the different zones.

Slovenia has not said what it will do if Croatia rejects the ruling. But it argues that the court's award will be "legally binding on both sides".

It instead will appeal to the EU, which it hopes will put pressure on Croatia to respect international law.

"It's about the credibility of the EU," a Slovenian diplomat said, adding that his government hoped that EU institutions "will stand up for international law and not play politics".

"The least we would expect is that the European Commission will take a clear position," he said.

Ahead of the ruling, however, the EU Commission has tried not to be dragged into the dispute between two member states.

The EU executive has "no competence on border disputes between member states," a spokesperson told EUobserver. "The commission supports amicable solutions between member states".

While the commission says that it is "not party to the arbitration agreement," Ljubljana insists that the 2009 deal was brokered by the enlargement commissioner at the time, Oli Rehn, and signed by Sweden, which held the EU presidency.

EU baby

"It's not a bilateral issue, it's the EU's baby," the diplomat said. "It was a bilateral issue until the arbitration agreement was signed under the auspices of the EU."

Slovenia also expects other members states to take a position and it has started to gather supporters.

Last week, in a statement to a Slovenian media, the German embassy in Ljubljana said that "the preservation of the integrity of an international jurisdiction is in the interest of all states" and that EU member states have to "lead the way by giving a good example".

"The federal government assume that both EU partners (Croatia and Slovenia) will apply the ruling of the arbitration court within six month," the German embassy added, referring to the delay set by the procedure.

On Monday, Estonian foreign minister Sven Mikser, whose country will take the six-month EU presidency on Saturday (1 July), assured his Slovenian colleague Karl Erjavec of Estonia's "support for respect of international law".

The ruling, and the potential tension, over a border that results from the breaking-up of Yugoslavia, comes at a time when nationalist rhetoric, government instability and suspected Russian meddling have raised EU concerns about the situation in the Western Balkans.

"The arbitration agreement was hailed as an example for other border disputes in the region," the Slovenian diplomat pointed out, outlining Ljubljana's political argument to the EU.

He insisted that "a failure to implement the ruling would have very negative repercussions for order border issues and postpone enlargements".