It would be up to the German justice system to determine whether such politicians should face jail time, the court said, after being asked to rule on a long-standing dispute between environmental activists and the state government of Bavaria.

In a legal tug-of-war stretching back to 2012, environmental group Deutsche Umwelthilfe (DUH) is attempting to force the Bavarian government to implement measures against air pollution in the state capital Munich.

Both activists and the judiciary have claimed the Bavarian government is flagrantly ignoring a 2014 Munich court decision demanding a plan of action to include a city ban for diesel-fuelled vehicles.

Thursday's ECJ opinion, though not legally binding, could have implications for leading politicians in the Bavarian sister party of Chancellor Angela Merkel's ruling Christian Democrats.

The ECJ said any jail sentence would require “a national legal basis which is sufficiently accessible, precise and foreseeable in its application”.

It added that such punishment must be “proportionate”.

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The court's advocate general had said in November that no such legal basis appeared to exist in Germany.

The Bavarian higher administrative court referred the case to the ECJ in November 2018, saying that “high-ranking political figures (had) made it clear, both publicly and to the court, that they would not fulfil their
responsibilities.”

Saying a €4,000 fine had proved “inefficient”, it asked the magistrates in Luxembourg to advise on the legality of threatening lawmakers with imprisonment.

In Thursday's ruling the ECJ recalled that the “referring court found that ordering the payment of financial penalties was not liable to result” in a change in conduct since the fine would be credited as income for Bavaria and thus “not result in any economic loss”.

It said incarceration should be a recourse “only where there are no less restrictive” measures such as stiffer, renewable fines whose payment “does not ultimately benefit the budget from which they are funded”.