The letter, which is the first step in Slovenia's plan to take Croatia to the EU's Court of Justice, was handed by Slovenia's Ambassador to the EU Janez Lenarčič to Clara Martinez Alberola, the new head of the Commission president's cabinet, Slovenia's mission to the EU said.

The letter's content remains confidential, but the information disclosed so far suggests it lists concrete instances of how EU law has been violated because of Croatia's refusal to implement the border between the two countries as set by an arbitral tribunal on 29 June 2017.

Including the documents attached, the letter unofficially runs to more than a hundred pages.

The European Commission now has three months to respond to the letter and to decide whether it will trigger procedure against Croatia before the EU court. If not, Slovenia will resume the procedure itself.

Slovenia's outgoing government adopted the letter on Wednesday just before PM Miro Cerar announced his resignation. Also adopted was a decree empowering the government to file a lawsuit against Croatia unless the step is taken by the Commission and Croatia continues to deny the validity of the arbitration award.

The Commission confirmed it had received the letter and said it would study it.

This is a legal dispute between member states on the basis of Article 259 of the Lisbon Treaty, and the Commission stands ready to mediate, the Commission said.

The Commission is ready to mediate as it did in earlier legal disputes between member states, for example last year in the Almaraz case between Spain and Portugal.

The Commission said that its college had discussed the case on 4 July and 20 December 2017 and the Commission's position on the border arbitration decision was well known.

Asked about what happens now, the Commission referred to Article "which sets out very clearly the several stages that need to be followed, including oral and written observations by the parties involved."