LJUBLJANA (Reuters) - Slovenia will take neighboring Croatia to court because Zagreb is unwilling to implement an international arbitration ruling on the border between the two European Union members, Prime Minister Miro Cerar said on Monday.

Slovenia’s Prime Minister Miro Cerar arrives at a European Union leaders summit in Brussels, Belgium, March 22, 2018. REUTERS/Francois Lenoir

The case against Croatia will be filed at the European Court of Justice in Luxembourg in July, he said. Slovenia and Croatia both declared independence from the former Yugoslavia in 1991 but have so far failed to agree on parts of their common sea and land border.

“The government will on Thursday confirm the decision to file the suit ... I expect the suit will be filed by the middle of July,” Cerar told a news conference.

A year ago the Permanent Court of Arbitration in the Hague ruled that Slovenia should have “uninterrupted access” to the international waters in the Adriatic Sea.

The dispute has chiefly affected fishermen from both countries who are being fined by the other state for illegally crossing the border.

The Hague court’s ruling is final and binding but Zagreb has said it is not valid because Croatia withdrew from the arbitration process in 2015.

This followed the leaking of a recording in which a Slovenian judge on the panel improperly exchanged confidential information with the Ljubljana government. The judge resigned and the court continued with the case, saying the incident had not compromised its ability to reach a verdict.

Slovenia asked the European Commission in March to issue its opinion on the matter but earlier on Monday Brussels said it would not do so.

“I am very worried about the silence of the Commission ... I believe politics prevailed over the law in this case,” said Cerar, adding the rule of law is “one of the fundamentals of the European Union”.

Croatia believes the dispute should be solved in bilateral talks although the two countries turned to arbitration in 2009 after failing to resolve the matter themselves in almost two decades.

The dispute has prompted a change in the EU’s expansion strategy, which was released in February and states that countries will no longer be able to join the bloc before resolving their border issues.

Another former Yugoslav republic, Macedonia, signed a deal with neighboring Greece on Sunday to settle a long-running dispute over its name. The agreement paves the way for the country’s possible admission to the EU and NATO.