Croatia is not involved in only one border dispute, but several more with Serbia, Bosnia-Herzegovina and Montenegro, the paper notes, warning that the Commission's position on EU enlargement would give Croatia more leverage to extort other countries.

"Insisting on resolving border issues beforehand seems reasonable at first glance but in practice in the Balkans those who already do not stick to agreements will get even more opportunities to extort.

This position could "bring exactly what [EU Commission President Jean-Claude] Juncker does not want in the 'very complicated, very sensitive' Western Balkans: importing instability from this region into the EU".

Juncker, who says that the Slovenian-Croatian border dispute causes him pain is afraid that conflicts from the early 1990s could be reignited overnight.

But in the Gulf of Piran and other border areas the European rule of law is already being tested, the paper says under the headline Sorrows of Old Juncker, adding that this is not something to be played with.