The European Commission President said the ongoing issue between Slovenia and Croatia was not merely a bilateral problem but one that impacted all of the EU.
Speaking at a press conference with the Slovenian President Borut Pahor, Jean-Claude Juncker said: “The future enlargement of the EU to Western Balkans states is in the hands of Slovenia and Croatia.
“This is not only a bilateral problem, this is a problem that impacts the whole European Union.”
He added the deadlock between the two EU member states was “impacting the perspective of Western Balkan states to become members of the European Union.”
In the long-running dispute, Croatia is refusing to implement a ruling handed down by the Permanent Court of Arbitration in The Hague last summer over the route of its 416 mile border with Slovenia.
The two countries have been at odds over the issue since the break-up of Yugoslavia in the 1990s. Slovenia joined the EU in 2004.
Croatia’s accession in 2013 was, at the time, made conditional on its acceptance of international arbitration.
And Croatia claims the process was marred by the links a Slovenian judge in the case had with the country’s government.
The court, which dismissed the judge, said the incident did not impede its work.
Tensions have also escalated due to several Croatian fishing boats entering Slovenian waters in the Bay of Kiran, one of the most hotly disputed areas between the two countries.
Speaking alongside Mr Juncker, Slovenian president Pahor said he wanted the Commission “to become more actively involved in implementing the arbitration award.”
He said the EU executive had a “political, legal, and moral duty to intervene.”
Mr Pahor said: “This is a question of credibility.”
Referring to Balkan states – among which Serbia and Montenegro have already started formal accession negotiations – the Commission chief added “they have to agree between themselves before they become member states.”
Mr Juncker also noted “nobody” in the EU knows what the dispute is about and that leaders in other EU countries “have very little interest” in the issue.
And the EU chief repeated the Commission’s willingness to mediate between Slovenia and Croatia but gave no detail on when or how his services could do it.
He prepared the ground for discussion by saying Slovenia and Croatia still enjoyed “friendly relations” and that the differences between them were “tiny”.
Mr Juncker said: “There is room for a negotiated solution.”