Jürgen Resch is upending the German car industry— one court case at a time. The white-haired veteran of the country’s environmental movement is behind a sweeping legal campaign to uphold air quality by imposing driving bans in German cities. His main target: diesel cars made by the likes of Volkswagen, Daimler and BMW. He has attracted both admiration and opprobrium but there is no dispute over his record: an unbroken string of courtroom victories, millions of furious vehicle owners and an industry reeling from reputational and financial blows, with potential losses running to billions of euros.

At a time when German carmakers are fighting President Donald Trump’s threat to hit their exports — leaders of the three leading manufacturers went to Washington this month to try to assuage US concerns — Mr Resch’s campaign has exposed them to a wearying domestic battle. But Mr Resch, the head of campaign group Environmental Action Germany (DUH), insisted in an interview that he simply wanted to uphold the law. “We have the strange situation that we as an [environmental organization are not fighting for tighter limits and tougher standards. We are fighting to ensure that existing law is applied,” he said in an interview.

“We are trying to make sure that the state sticks to its own laws.” According to official data, 65 German cities fail EU air quality standards due to excessive levels of nitrogen dioxide (NO2), a leading cause of respiratory disease. Diesel cars, which typically emit significantly more NO2 than petrol-driven vehicles, are seen as a prime cause.

What marks out Mr Resch as an environmental campaigner is that his battles take place mainly in court, where the DUH has challenged dozens of local governments to enforce the nitrogen dioxide limit and ban diesel cars from cities’ most heavily polluted streets. The DUH has won 12 rulings ordering cities to impose such driving bans, including in Berlin, Cologne, Frankfurt, Stuttgart, Essen and Mainz. In total the group has filed 30 cases, while another four cases are to be filed.