The EU's highest court ruled Wednesday that Ireland can refuse to hand over an alleged drug dealer to Poland if its judiciary determines he may not receive a fair trial there, in a move that will inflame the ongoing fight between Brussels and Warsaw over rule of law.

The judgment opens the way for Ireland's judiciary to deliver a damning verdict on Poland's legal system. If Irish courts go ahead and block the European Arrest Warrant, they will in effect be deciding that Poland's judiciary no long functions as a normal part of the EU's justice system — dealing a blow to Poland's ruling Law and Justice party.

The case was referred to the European Court of Justice after Artur Celmer, a Polish national, was arrested in Ireland under a European Arrest Warrant for drug trafficking. Celmer’s lawyers opposed their client’s extradition to Poland following significant changes to Poland’s judiciary, which they claimed undermine its independence and jeopardize cooperation on warrants.

Wednesday’s ruling by the ECJ says it is up to the Irish court that issued the arrest warrant to assess whether there is a “real” risk “linked to a lack of independence of the jurisdictions” that the defendant would not be able to get a fair trial in Poland “because of failures” in its rule of law.

The ruling underlined that refusing to execute a European Arrest Warrant is “an exception,” and should be subjected to a “strict interpretation.”

To be able to block the arrest warrant, the ruling added, the Irish court will need to make a two-step assessement, which should be done “on the grounds of objective, accurate and duly updated elements." First, the judicial authority should note that there is a real risk of “inhuman or degrading treatment in the member state” because of "notably systemic failures.”

Second, that authority should make sure there are “serious and proven” reasons to believe "that the person who is subjected to the warrant will be exposed to such risk."

Polish authorities were very critical of the Irish court’s decision to involve the ECJ. But the country's justice minster told reporters the judgement was a "serious defeat” for the Irish court.

"[The ECJ] advised the Irish court in a subtle way that there are procedures, which are compulsory and undertaken since years in such cases. If there is any doubt about them, those procedures should be verified and proven every single time,” Ziobro said.

In December, the European Commission launched so-called Article 7 proceedings against Poland over changes to the way that judges can be dismissed, which Brussels believes amounts to political interference. The process could result in the country losing its voting rights as an EU member. If Ireland's courts determine in Celmer’s case that there is a systemic threat to the rule of law in Poland, that could add weight to a move for the Council of the EU to endorse the Article 7 process.

The ECJ ruling largely mirrors an initial opinion issued by the court's advocate general in June, but the judgment on Wednesday added qualifiers. It underlines that refusing to execute a European Arrest Warrant is “an exception,” and should be subjected to a “strict interpretation.”

Jago Russell, chief executive of Fair Trials, a U.K. registered global criminal justice watchdog, said he "welcomed the recognition that there are limits to mutual trust in Europe and that threats to fair trial rights can prevent extradition. "

However, he added, "the court’s approach will be incredibly hard to apply in practice.”

Philippe Lamberts, president of the Greens/EFA group in the European Parliament said the ruling was a warning to the Polish government about its "path away from European democratic values."

"The erosion of the rule of law weakens mutual trust between EU member states, which is clear by the fact that the ECJ refuses to uphold the European Arrest Warrant for requests from countries where an independent judicial system is under attack. The Polish government must reverse its recent changes to the court system and guarantee fundamental rights such as the right to a fair trial for all,” he said.