A German court will rule on Thursday whether major cities can ban heavily polluting diesel cars, potentially hitting the resale value of 15 million vehicles in Europe’s largest car market and forcing carmakers to pay for costly modifications.

Diesel-engine cars have been in the spotlight since Volkswagen admitted in 2015 to cheating U.S. exhaust tests, triggering a global backlash against emissions that contain particulate matter and nitrogen oxide (NOx) and are known to cause respiratory disease.

Environmental group DUH sued city authorities in Stuttgart, in Germany’s carmaking heartland, and in Duesseldorf over levels of particulate matter exceeding European Union limits.

Local courts ordered cities to bar diesel cars that did not conform to the latest standards on days when pollution is heavy.

The German states have appealed against such rulings, leaving the country’s highest federal administrative court in Leipzig to rule on whether they are legal.

The issue goes beyond Germany, Europe’s largest economy and home to some of the continent’s biggest carmakers.

Authorities in Paris, Madrid, Mexico City and Athens have said they plan to ban diesel vehicles from city centers by 2025, while the mayor of Copenhagen wants to ban new diesel cars from entering the city as soon as next year.

France and Britain will ban new petrol and diesel cars by 2040 in a shift to electric vehicles.

Read more on Reuters.