The letter sent on Wednesday comes after the tribunal arbitrating on the border dispute appointed last week Rolf Einar Fife of Norway and Nicolas Michel of Switzerland as new arbitrators, making the tribunal complete.

The Slovenian and Croatian arbitrators resigned in the space of several days in late July after a Croatian media outlet published purported recordings of the Slovenian arbitrator Jernej Sekolec holding conversations with one of the country's agents.

In the letter, Juncker and Timmermans said they were happy that two new arbitrators had been appointed and that the tribunal was operational again.

"While the setting of borders between member states does not fall directly within the competences of the Union, the European Commission reiterates that open border conflicts between member states shall be resolved promptly, not least because these can have an impact on the application of the EU law," the letter says.

The European Commission is confident that both member states will act in the spirit of sincere cooperation, and trust that arbitration process will "provide for appropriate mechanisms to deal with all issues that have arisen."

The officials expect that the dispute will be settled in full respect of arbitration agreement, and that the parties will respect the decision of the tribunal.

Confirming that he and Juncker had signed the letter, Timmermans told the press in Brussels on Thursday that it is "obvious that the Commission would like member states to solve border disputes because that makes it easier for Commission law to be applied correctly in all member states, that he have clarity about where the border is."