Roland Vaubel, a former lecturer at the University of Mannheim in southwestern Germany, said the conditionality mechanism, which aims to cut cash payments to member states found in breach of the bloc's rules on democracy, contravenes the EU's treaties, the PAP news agency reported.
It said Vaubel was a political economist specialising in international monetary policy and international organisations.
It quoted him as saying that the European Commission, the EU's executive, was "seeking to expand its powers in an unacceptable way ... under the excuse of fighting for the rule of law."
The arguments came after Poland’s Justice Minister Zbigniew Ziobro last week said he would ask the country’s Constitutional Tribunal to declare that the European Union’s rule-of-law conditionality mechanism is incompatible with the national constitution.
Ziobro told reporters at the time that "extensive legal analysis" had shown “beyond the slightest doubt” that the EU mechanism was “radically at odds” with the Polish constitution.
He slammed the European rule-of-law tool as incompatible with “both the letter and the spirit of EU treaties and the Polish constitution.”
He argued that EU bodies "have decided to overstep their powers in this way."
According to Ziobro, the rule-of-law conditionality mechanism "is by its nature very dangerous” because "it allows the European Commission to exercise blackmail or even enormous economic violence, for political, arbitrary reasons, and outside any control."
The mechanism, which ties access to EU funds to compliance with rule-of-law principles, was agreed by negotiators from the European Parliament and the German presidency of the EU in November last year and then approved by the ambassadors of the member states, the PAP news agency reported.
The push met with criticism at the time from Poland and Hungary, with Polish Prime Minister Mateusz Morawiecki sending a letter to EU leaders to oppose “arbitrary and politically motivated criteria.”
Following negotiations, EU leaders finally approved the mechanism at a summit in Brussels in December last year.
Ziobro last week accused Brussels of seeking to "incapacitate Poland and Polish democracy” after the European Commission launched legal proceedings against Warsaw for questioning the primacy of EU law.
Germany's Vaubel has previously been quoted as saying that Poland’s Constitutional Tribunal was right in ruling that only national courts could interpret the limits of the powers of EU institutions.