International judges on Thursday awarded Slovenia key access to international waters off the Croatian coast, sparking a furious reaction from Zagreb which said it would refuse to implement the ruling.
In a complex 300-page judgement, the judges ruled Slovenia should have "a junction area" with international waters, allowing "freedom of communication" to all ships, civilian and military, seeking access to Slovenia.
They determined that the "junction between the Slovenian territorial sea and the 'High Sea' is an area in which ships and aircraft enjoy essentially the same rights of access to and from Slovenia as they enjoy on the high seas," presiding judge Gilbert Guillaume said, in a two-hour long judgement.
In Ljubljana, Slovenian Prime Minister Miro Cerar hailed the ruling as a "historic moment for Slovenia," saying the judgement "is definitive and must be applied on both countries, Slovenia and Croatia".
The Slovenian leader said he would call his Croatian counterpart during the day to "begin dialogue on implementing the decision".
"Slovenia will do nothing to harm relations between our countries or our citizens," he said.
But Croatian Prime Minister Andrej Plenkovic said his country will refuse to implement the ruling, saying it was "not obliging us in any way".
The sea corridor will be about 2.5 nautical miles wide, and lies about 12 nautical miles beyond the territorial limits of both Croatia and Italy.
But in the case which was first lodged with the Permanent Court of Arbitration in 2009, the court rejected Ljubljana's claim that the whole of the southwestern Piran bay was Slovenian territory.
Instead they determined the maritime border "shall be a straight line joining a point in the middle of the channel of the St Odoric Canal" heading straight into the bay from the mouth of the Dragonja River.
The area under dispute is a tranquil bay on the northern Adriatic Sea, where the medieval buildings of the southwestern Slovenian town of Piran tumble down to a sleepy port.
But the bay is also shared by Croatia, and the dispute over where the sea borders should be drawn has poisoned relations between the neighbours since they both declared independence from Yugoslavia in 1991.
In 2009, the two countries signed an EU-backed deal to allow the Permanent Court of Arbitration in The Hague to solve the long-standing dispute over the 13 square kilometres (five square miles) of largely uninhabited land and Piran Bay.
Slovenia, which has just 46 kilometres (29 miles) of coastline, had argued its access to international waters was at stake because Croatia, whose coast stretches for 1,700 kilometres, wanted the border to be drawn down the middle of the disputed bay.
The judges stressed that the aim of the corridor was to "guarantee both the integrity of Croatia's territorial sea and Slovenia's freedoms of communication between its territory and the high seas."
Zagreb had only agreed to join the proceedings after Ljubljana lifted its veto in 2009 to Croatia's accession to the European Union. But it pulled out again in 2014 following a phone tapping scandal.
A Slovenian judge from the tribunal and a Ljubljana official were recorded discussing tactics for a ruling favourable to Slovenia.
Observers have warned that if Zagreb does not comply with the ruling it could further strain already tense relations with Slovenia, which is Croatia's key entry point into the passport-free Schengen zone.