The proposal comes after Slovenia presented to the European Commission last month a letter in which it set out its case against Croatia.

Slovenia plans to take Croatia to the EU's Court of Justice over its refusal to implement the border between the two countries as set by an arbitral tribunal on 29 June 2017.

The Commission would not comment on further steps in the case, saying only that it was important to remain in line with Article 259 of the Lisbon Treaty. The article provides an opportunity to address "legal conflicts within the family", said Commission spokesperson Margaritis Schinas.

He reiterated the well-known position of the European Commission that the arbitration dispute was a legal dispute between two EU member states and that Brussels was willing to mediate in the matter as it had done before.

Schinas pointed to the case of Almaraz, a Portuguese-Spanish dispute over a nuclear waste repository, which was resolved after mediation by Commission President Jean-Claude Juncker.

The Commission says that Almaraz is similar to the Slovenian-Croatian dispute, but Slovenia claims that the cases are not comparable as there had been no prior ruling on the Portuguese-Spanish case.

According to unofficial information, the Commission proposed the date to both sides several days ago and at the same time called on Croatia to submit by 17 April a written response to Slovenia's letter, which was presented to the Commission on 16 March in line with Article 259 of the Lisbon Treaty.

Oral submission of observations under Article 259 is the basis for an EU member state to take another member state to court. It will be held at the level of experts and behind closed doors.

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

It remains unclear if the Commission will present its opinion at all, with the oral hearing on 2 May the first procedural step that will be the basis for all further steps, according to unofficial information.

Furthermore, the Commission can decide to hold several oral hearings, with today's comments indicating that the Commission wants to tackle the issue outside court.

On the same day, the Commission will unveil its key proposal on the multi-year EU budget after 2020.