A Polish judge with access to classified information who fled to Belarus was on Thursday stripped of his legal immunity, opening the way for potential espionage charges.

Belarus announced Monday that Tomasz Szmydt had entered the Moscow-allied state, with whom Poland has tense relations, particularly since the start of the war in Ukraine.

Szmydt was close to Poland's right-wing Law and Justice (PiS) party, which lost power to a pro-European Union coalition last year.

Prosecutors launched an inquiry into possible espionage this week, and Prime Minister Donald Tusk said Szmydt's case bore the mark of "high treason".

On Thursday, a disciplinary court "gave authorisation for Tomasz Szmydt to be held criminally liable," Anna Adamiak, spokeswoman for the prosecutor general, said in a statement.

"This decision... will lead to the judge's loss of immunity in this case, which will allow for charges to be brought against him," she added, citing the law on espionage.

Adamiak said evidence "clearly shows that Tomasz Szmydt participated in hybrid warfare ... conducted by Belarus and Russia against Poland".

His "actions were meant to undermine Poland's position in the world as well as its foreign policy and to provoke internal tensions".

Justice Minister Adam Bodnar said earlier that loss of immunity would allow for an arrest warrant and Interpol notice to be issued, though he acknowledged that Polish services would not be able to detain the judge in Belarus.

Szmydt has ruled on cases involving security certificates for secret classified information concerning NATO, the EU and the European Space Agency, according to media reports.

He has also worked at the justice ministry and the trade and companies registry.

He took part in a programme by Russian presenter Vladimir Soloviov, described by the US State Department as "the most energetic pro-Kremlin propagandist".

The judge and his ex-wife took part in an online campaign attacking judges who spoke out against legal reforms introduced by the PiS when it was in power.