EU leaders in December signed Croatia's accession treaty, trumpeting the move as a sign the EU's enlargement programme is alive and well despite the euro crisis.
But now Slovenia is threatening to block it over a €172 million bank dispute.
"Only after [the dispute] is solved in accordance with international law, will we be able to start the ratification procedure for Croatia's accession to the European Union," Jozef Horvat, an MP in the ruling centre-right SDS party and the head of the parliament's foreign affairs committee told press in Ljubljana on Thursday (20 September).
Commenting on bilateral talks on the issue, Slovenia's foreign minister, Karel Erjavec, noted that "[Slovenia's negotiator] has a mandate until the end of the year, if a solution is not reached by then, that means no bilateral solution can be found."
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The dispute concerns the now defunct Ljubljanska Banka and the break-up of former Yugoslavia.
When Ljubljanska Banka went bankrupt in the 1990s, the €172 million, which belonged to some 130,000 Croatian savers, was folded into Slovenia's national debt.
Two Croatian banks have filed lawsuits against Ljubljanska Banka's reincarnation, the Nova Ljubljanska Banka, to get the money back.
But Slovenia says the case should be settled as part of a wider Yugoslav-break-up deal being brokered by the Bank of International Settlements in Basel, Switzerland.
To add insult to injury, Slovenia in June gave Nova Ljubljanska Banka €382 million to keep it afloat.
Meanwhile, Horvat's statement will come as no surprise to Brussels.
The Slovenian foreign minister already annoyed EU colleagues at a meeting in the EU capital in July when he told press: "I personally wish Croatia would join the EU as soon as possible and that we ratify the accord ... But the precondition for that is that we solve the question of LB [Ljubljanska Banka]."
He repeated his position at an informal meeting of EU foreign ministers in Cyprus in September.
An EU source told EUobserver: "Nobody in the EU likes it when EU matters are linked with bilateral problems ... If everybody did it, it would overshadow enlargement as a whole. Everyone could take something out of the bag against their neighbours and it would pollute the atmosphere."
The contact noted that €172 million might not sound like much, but given Slovenia's wobbly financial situation "every euro counts."
The source added that "it is not the first time that Slovenia has done this [blocked an EU initiative for its own reasons]."
In 2008, Slovenia threatened to stop Croatia accession talks over a maritime border dispute.
Earlier this year, it vetoed EU sanctions on Belarus in order to protect a Slovenian firm's hotel contract in Minsk.
Croatia is supposed to join the EU on 1 July 2013 if everybody ratifies its accession treaty on time. Fourteen EU countries and Croatia itself have done it so far.