In the judgment in Slovenia v Croatia (C-457/18), delivered on 31 January 2020, the Grand Chamber of the Court has declared that it lacks jurisdiction to rule on the action brought by Slovenia, on the basis of Article 259 TFEU, for a declaration that Croatia has failed to fulfil its obligations under EU law, by not having complied with obligations stemming for the latter Member State from an arbitration agreement concluded with Slovenia that was intended to resolve the border dispute between those two States and from an arbitration award defining the sea and land borders between the two States.
However, the Court stated that its lack of jurisdiction is without prejudice to any obligation arising — for each of those two Member States, both in their reciprocal relations and vis-à-vis the European Union and the other Member States — from Article 4(3) TEU to strive sincerely to bring about a definitive legal solution consistent with international law, in order to ensure the effective and unhindered application of EU law in the areas concerned.
For more detailed information see Court of Justice of the European Union PRESS RELEASE No 9/20: https://curia.europa.eu/jcms/upload/docs/application/pdf/2020-01/cp200009en.pdf
Full text of the judgement is available on the following link: http://curia.europa.eu/juris/documents.jsf?num=C-457/18
