The European Court of Justice will advise whether to impose heavy penalties over vehicle pollution.
The European Court of Justice (ECJ) will start examining on Tuesday whether German courts should give prison sentences to politicians who don't enforce bans on heavily-polluting cars.
The case stems from a long-standing dispute between environmental activists and the state government of Bavaria. The environmental group Deutsche Umwelthilfe (DUH) is trying to force the Bavarian government to implement measures against air pollution in the state capital of Munich, where nitrogen dioxide levels exceed EU limits.
In 2014, a Munich court demanded a plan of action from the state government for a city ban for diesel cars. Environmental activists claim the Bavarian government is ignoring this ruling on purpose.
"We are asking that air pollution limits be respected," DUH chairman Juergen Resch told AFP.
November last year, the Bavarian higher administration court referred the case to the ECJ because "high-ranking political figures" had "made it clear, both publicly and to the court, that they would not fulfil their responsibilities."
The court added that a 4,000 euro fine had been "inefficient" and wanted the ECJ to advise on the legality of imposing a prison sentence to MPs who didn't enforce the ban.
The ECJ's decision though not legally binding could have serious implications for the Bavarian sister-party of Chancellor Angela Merkel's ruling Christian Democrats who oppose the bans.
Ugo Taddei, a clean air lawyer at NGO ClientEarth, said the ECJ hearing was important because it would "give a clear interpretation, a clear indication to German courts about what they need to do when they run into a situation like this."
However, even if the ECJ rules that MPs can be given prison sentences, it will be up to Bavarian courts to decide what to do.
For government authorities, a diesel ban remains a bad option.
"Driving bans are a bad solution," a spokesperson for the state environment ministry told AFP.
"The air quality in Bavaria is improving, so the measures taken so far are working," said the ministry, pointing to investment in software updates and cycling and public transport infrastructure.
Back in February 2018, Germany's highest federal administrative court gave the green light to big German cities to ban polluting vehicles — a major blow to Europe's largest car market.
The decision came after German states appealed against bans imposed by local courts in Stuttgart and Duesseldorf.
The cases were brought by environmentalists concerned about poor air quality.
Merkel's government, which has come under fire for its close ties to the car industry, had lobbied against a ban, fearing it could anger millions of drivers and disrupt traffic in cities, with public transport not in a position to take up the slack.
Taddei added that the ECJ ruling was important because "it would be binding for all judges in the EU27 member states across the EU".