The European Union's chief prosecutor has accused Slovenia of interfering in the EU's justice system.
Slovenia was the sole EU member state that failed to name two prosecutors to join the European Public Prosecutor's Office.
The office, which began work in June, aims to independently investigate crimes against the EU budget, such as corruption and serious cross-border value-added tax fraud.
"The fact that a member state is interfering with the proper functioning of an EU judicial institution sets a very dangerous precedent," Chief Prosecutor Laura Kovesi told EU lawmakers.
Kovesi added that Slovenia's failure to nominate prosecutors to the agency has left "a prosecution gap in the EPPO zone."
The process of selecting the delegates was carried out by a council of Slovenian prosecutors, but Prime Minister Janez Jansa said in July that the procedure "was not carried out correctly".
Jansa -- who currently holds the EU's rotating presidency -- said that only two candidates were proposed for the posts, "even though several candidates came forward".
Jansa has accused his country's own justice system of moving too slowly on fraud and corruption.
European Commission President Ursula von der Leyen said at the start of Slovenia's current term at the EU's helm that the public prosecutor's office is "a crucial component to protect EU taxpayers' money".
But Kosevi said her office now has to work as if it "did not exist in Slovenia".
"In this situation, how can Slovenia ensure proper and complete supervision of bodies responsible for the management and control of Union funds?" she asked
"How can Slovenia guarantee effective judicial follow-up of all the detected fraudulent irregularities? Is the EU budget properly protected?''
A spokesman for the Commission said that Brussels has had several exchanges of letters and discussions with the Slovenian authorities.
"This is not a sustainable situation," said the spokesman, Christian Wigand.
Justice Commissioner Didier Reynders has threatened to take Slovenia before Europe's top court if it does not respect its obligations.
But the Commission is unlikely to move before a Slovenian administrative court rules on complaints by the two prosecutors whose candidacies were refused.