A top adviser to Europe's highest court has said it is up to an Irish judge to determine whether to extradite a suspected Polish drugs trafficker.
It follows European concern over alleged interference by the government in Warsaw in domestic judicial processes.
Ireland's High Court referred the case of Artur Celmer, arrested in May last year in Dublin using a European Arrest Warrant, to the Court of Justice of the EU in Luxembourg.
UK membership after Brexit of the pan-European policing scheme which allows speedy extradition of suspects is part of withdrawal negotiations.
The outcome could have a critical impact on tackling criminality across the island of Ireland.
On Thursday, a statement on behalf of the advocate general to the Court of Justice Evgeni Tanchev said: "The execution of a European arrest warrant must be postponed where the competent judicial authority finds not only that there is a real risk of flagrant denial of justice on account of deficiencies in the system of justice of the issuing member state but also that the person who is the subject of the warrant is exposed to such a risk."
According to the advocate general's independent advice, it cannot be ruled out that lack of independence of the courts of the issuing member state may, in principle, amount to a flagrant denial of justice.
The official statement added: "However, in order for that to be the case, the lack of independence must be so serious that it destroys the fairness of the trial.
"It is for the Irish court to determine, on the basis of those considerations, whether, in the case in point, the alleged lack of independence of the Polish courts is so serious that it destroys the fairness of the trial and accordingly amounts to a flagrant denial of justice.
"The Irish court must, to that end, rely on information which is objective, reliable, specific and properly updated and demonstrates that the deficiencies affecting the Polish system of justice exist.
"In that regard, the Commission's reasoned proposal can be taken into account, provided that the Irish court informs itself of any changes in the situation in Poland subsequent to that document."
Celmer is the subject of three European Arrest Warrants issued by Polish courts over alleged illicit trafficking in narcotic drugs.
He fought extradition over fears that, on account of the reforms of the Polish system of justice, he runs a real risk of not receiving a fair trial in Poland.
The European Commission has taken unprecedented disciplinary measures against Poland and said its judicial changes - which include forcing some senior judges to retire - endangered the rule of law.
Thursday's opinion from Mr Tanchev said: "In order to determine whether the individual concerned is exposed to such a risk, the executing judicial authority must take account of the particular circumstances relating both to that person and to the offence in respect of which he is being prosecuted or has been convicted."
The opinion is not binding on the Court of Justice.
Judgment will be given at a later date.