Croat fisherman in disputed Piran Bay. Photo: Atonio Bat/EPA

Slovenia on Friday submitted to the European Commission a letter of complaint against neighbouring Croatia for its refusal to abide by an arbitration court ruling on a border dispute in the Adriatic Sea.

Slovenia accused Croatia of violating the EU Common Fisheries Policy by sending police escorts to guard fishermen in contested waters and preventing Slovenian inspectors from boarding the vessels.

Relations between the two neighbors deteriorated in June 2017 when the Permanent Court of Arbitration in The Hague ruled in favour of Slovenia over a territorial dispute about waters in the northern Adriatic’s Piran Gulf.

The letter submitted by Slovenia’s representative to the EC contained 100 pages of alleged violations of European law, which the Slovenian government says stem from Croatia’s refusal to abide by the arbitration court’s ruling.

Under the EU’s Treaty of Lisbon, the European Commission, the EU’s executive body, must respond to the complaint within three months.

On Friday afternoon, Croatia’s foreign ministry issued a statement saying that Slovenia had no ground for launching a complaint against Croatia.

“We believe Slovenia has no grounds nor reasons to launch any kind of a procedure against Croatia in front of the EU court, for alleged breach of EU rights.

“As we said several times before, Croatia will know how to respond [to moves like this],” the statement for Croatian news agency Hina said.

Outgoing Slovenian Prime Minister Miro Cerar, whose cabinet approved the letter on Wednesday, hours before his resignation, told Slovenian media that Slovenia will sue Croatia in the European Court of Justice, which rules on questions of EU law, if the Commission does not act on Slovenia’s complaint and if Zagreb does not comply with the arbitration ruling.

So far, the Croatian government has refused to follow that ruling. It charges that the arbitration process was compromised.

In July 2015, Croatia revealed recordings of alleged unauthorised phone conversations between Jernej Sekolec, a Slovenian judge on the Permanent Court of Arbitration, and Simona Drenik, the Slovenian government’s representative to the Court.

Last September, Slovenia vetoed Croatia’s accession to the Organisation for Economic Co-operation and Development as a direct consequence of this unresolved issue between the two states.

After months of rows and cancelled meetings, Slovenian Prime Minister Cerar visited Zagreb in December 2017 in a move expected to resolve the territorial dispute, but the trip resulted in no progress.

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NOTE: This article was updated on March 16 to include a reaction from Croatian foreign ministry.