Cerar said he would call a meeting of deputy group leaders on Wednesday to inform them of the filing of the lawsuit.
The prime minister expressed disappointment with the European Commission's decision not to give an opinion on the matter. The Commission decided for this option today and called on the two countries to find a solution, offering to mediate.
"I'm very worried by the silence of the European Commission, since it is not performing its duties as the guardian of EU treaties," Cerar pointed out.
"It's a very dangerous precedence for the future of the EU. These are not simple matters, they also concern the Schengen zone," the outgoing prime minister told the press.
He added that "in this case politics unfortunately prevailed over law" and that this is a scary prospect for any EU member which "will expect a fair position from the EU while at the same time fearing that politics will trample the law".
"The EU stands or falls on the rule of law. Thus it is important that Slovenia files this lawsuit ... that a standard is secured that needs to prevail in the EU if we want to prevent Europe from drifting into the wrong direction".
Cerar said the lawsuit would definitely be filed by mid-July. The government will take the decision on Thursday, he asserted, explaining the government, even though in a caretaker role now, had been authorised by parliament already in March to go ahead with the lawsuit.
He expects the EU Court of Justice to take the lawsuit very seriously and come to the conclusion that "Croatia needs to implement the arbitration decision immediately".
"I really don't see any more manoeuvring space for any kind of legal dodging after that," he added, while stressing Slovenian politics has been united on the issue.
Cerar went on to reiterate that the European Commission called last year for the implementation of the arbitration award and that Commission President Jean-Claude Juncker stated in September that the rule of law is not a choice in the EU but an obligation.
"It is merely for political reasons that the European Commission dismissed the legal arguments in this case," Cerar said.
He did not wish to comment on speculation that the Commission's European People's Party (EPP) members, politically affiliated with Croatia's leadership, were responsible for this.
Cerar also rejected Croatian media reports that the Commission's legal service indeed agreed with Slovenia in two of the four legal points raised against Croatia, saying the legal service "is agreeing with the legal positions of our government in all elements".
President Borut Pahor is also not happy with the Commission's stance. He believes it has missed an opportunity to politically and legally support efforts to implement the arbitration award.
Pahor, the father of the 2009 arbitration agreement between Slovenia and Croatia sponsored by the EU, said that the Slovenian-Croatian border dispute had been solved with the arbitration award and that the only dispute remaining was the one concerning its implementation.
If the Commission offered its opinion on the matter, this could speed up the implementation and have multiple consequences for the solving of bilateral issues between countries in SE Europe, he said.
"The European Commission did not meet the expectation to side with European and international law and thus give a clear signal that agreements and court decisions must be honoured."
Pahor is convinced that the border between Slovenia and Croatia as determined by the arbitration tribunal will be implemented sooner or later.
