The statement comes after it was revealed that the Commission had had an opinion from its legal department that vindicated Slovenia's position on arbitration but it was not formally discussed by the commissioners.

Bulc told TV Slovenija on Sunday that the document had not been discussed, but she also said it was not too late for this if Slovenia requested that the arbitration award be examined.

However, she also pointed out that it was the Commission president who decided whether an issue is put to a debate by all commissioners or not.

At today's press briefing in Brussels, the Commission's chief spokesman Margaritis Schinas reiterated the position that the Commission was not obligated to issue a report on the case, adding that no report was issued in four out of eight similar cases.

Responding to the legal opinion, some Slovenian MEPs were very critical of Commission President Jean-Claude Juncker, accusing him of undermining EU law, while others came to his defence.

Romana Tomc (EPP/SDS) said she had not been aware of the document, learning about it only from the media, but noted Juncker's EPP affiliation should not play a role.

Her SDS colleague Milan Zver said that if the Commission had not taken the document, if it existed, into account, this was not an EPP issue, but an issue for the Commission and Slovenian Commissioner Violeta Bulc to respond to.

Their EPP peer Lojze Peterle of the NSi explained he had not seen the document, but had been aware lawyers were working on a legal opinion.

However, it would be only right for the Commission to follow the legal opinion even if it had the right not to issue an opinion on Slovenia's lawsuit against Croatia.

"This is not about whether to side with Slovenia or Croatia, but whether to side with law. It would also be legally and politically right for the EPP to join the Commission's opinion that the ruling should be implemented, which has been expressed at least a dozen times."

Peterle also said the Commission could not condemn Hungarian PM Viktor Orban's alleged violations of EU law while turning the blind eye when it came to the arbitration tribunal. "This is about credibility. I'll talk to EPP senior officials about this," he told the STA.

Similarly, Franc Bogovič (EPP/SLS) intends to bring the issue up at the EPP group, but he would not agree the group was responsible for the document not being discussed.

He believes Commission Vice-President Frans Timmermans, who is in charge of the rule of law and comes from the Socialists&Democrats (S&D), is to be held accountable.

The fact that the Commission "has given up", clearly shows that Croatia has outplayed Slovenia's foreign policy led by the former prime minister and foreign minster, said Bogovič, who also learnt about the legal opinion from the media.

Harsher criticism was levelled at Juncker by non-EPP MEPs, with Tanja Fajon (S&D/SD) expecting Juncker at least to apologise and the Commission to discuss the legal opinion, "or else Juncker should think about stepping down".

Noting the Commission was the guardian of EU treaties and the rule of law was the basis of European integration, Fajon said those who undermine these foundations undermine Europeans' trust in European politicians.

She also warned about negative consequences for the Western Balkans if the Commission acts irresponsibly and in a biased manner when it comes to unresolved border disputes.

Igor Šoltes (Greens) said Juncker acted in an non-transparent and untrustworthy manner, totally ignoring EU law, and reproached him with "a Europe of different treatments".

Šoltes believes the Slovenian government should react resolutely, informing all EU members that Juncker had acted in a non-credible manner.