The magazine released an eight-page legal opinion, dated 14 May, 2018, which the Legal Service drew up for the Commission before the three-month period expired for a response to Slovenia's proposal to take Croatia to the EU court under Article 259 of the EU Treaty.

"The Legal Service considers that most of the heads of claim put forward by Slovenia in order to establish a breach of EU law by Croatia are established, and that a reasoned opinion should therefore be adopted pursuant to Article 259," the opinion concludes.

Summing up the Slovenia-Croatia border arbitration dispute, the Legal Service's opinion says that Slovenia's complaint is, in essence, that Croatia, by refusing to recognise the arbitration award, prevents Slovenia from fulfilling its obligations and enjoying its rights under EU law.

The Legal Service noted that the arbitration tribunal in 2016 rejected Croatia's claims that the arbitration process was compromised and that the arbitration agreement was no longer valid.

Thus the only relevant question for the EU is whether it should respect the decision of the international arbitration tribunal and take it into consideration in the application and interpretations of questions of EU law.

The Legal Service notes that the obligation to respect international law must include respect for legal principles such as "pacta sunt servanda" (agreements must be kept) and "res judicata" (respecting a matter already judged), that is including treaties such as the arbitration agreement and arbitration award.

"In this respect the fact that implementing measures may still have to be taken, as argued by Croatia, cannot mean that it would be possible to disregard the final award and the provisions of the arbitration agreement.

"The possible implementation would therefore only concern arrangements on the material demarcation of the border where this is needed (mainly on land) and the adjustment of internal legislation and regulation. In the marine areas the lines have been precisely determined by the award and this aspect would normally not even require any further arrangements on demarcation.

"In the light of the above, and without making any judgements of its own, the Commission must just observe that there is an arbitration agreement which has not been validly terminated and that there has been final determination of the borders between Slovenia and Croatia.

"Therefore the outcome of the arbitration procedure must be respected by the EU, and provisions of EU law must be interpreted in the light of it," the Legal Service's opinion reads.

The opinion also upholds Slovenia's claims of Croatia's infringements of the general duty of loyal cooperation, specific duties under the Common Fisheries Policy, specific duties under the Schengen Code and specific duties under Maritime Special Planning Directive.

Der Spiegel's Brussels correspondent Peter Müller said that that Commission President Jean-Claude Juncker was ignoring his own legal staff's advice, suggesting that this was to help Croatian Prime Minister Andrej Plenković, who is a member of the same political group.

While the European Commission and Croatia declined to comment on the report, the revelations invited a unanimous response from Slovenia's most senior officials, who reiterated that there was no alternative to the implementation of the border arbitration award.

President Borut Pahor commented that the European Commission's ignoring the opinion of its own Legal Service due to political reasons was a bad message for potential future border agreements in the Western Balkans.

Foreign Minister Miro Cerar said the document published by Der Speigel reaffirmed his assessment that the European Commission failed to do its job as a guardian of EU treaties.

"It all confirms that there's no other way but for both countries to set out to implement the award," Cerar said, adding that several months ago, when he still served as prime minister, Slovenia proposed forming a bilateral demarcation commission to make the life of people along the border easier.

Prime Minister Marjan Šarec said the European Commission's Legal Service ascertained that the rule of law must be respected, but that the Commission decided otherwise.

"We must insist on the rule of law," Šarec said, adding that the arbitration award must be implemented, even though "it wasn't as favourable for Slovenia as some may have thought".

Speaker Dejan Židan took his criticism further by saying that Juncker threatened the foundations of the EU through his politically-motivated actions, and that it was right that he should be leaving.

His party colleague, Dominika Švarc Pipan, an international law expert who has been appointed a state secretary at the Ministry of Justice, tweeted: "No wonder the Commission wouldn't make the opinion public or send it to Slovenia. It'll now be really hard to justify silence or pretend ignorance any longer."