BRUSSELS (Reuters) - Banning diesel cars in European cities could hamper automakers’ ability to invest in zero-emission vehicles, the European Union’s commissioner for industry has warned the bloc’s transport ministers.

A fuel nozzle is seen at a gas station in Berlin April 3, 2012. REUTERS/Tobias Schwarz

In a letter seen by Reuters, Commissioner Elzbieta Bienkowska said there would be no benefit in a collapse of the market for diesel cars and that the short-term focus should be on forcing carmakers to bring dangerous nitrogen oxide emissions into line with EU regulations.

“While I am convinced that we should rapidly head for zero-emission vehicles in Europe, policymakers and industry cannot have an interest in a rapid collapse of the diesel market in Europe as a result of local driving bans,” Bienkowska said.

“It would only deprive the industry of necessary funds to invest in zero-emissions vehicles,” she said in the letter, dated July 17.

Germany’s three major carmakers have invested heavily in diesel technology, which offers more efficient fuel burn and lower carbon dioxide emissions than gasoline-powered cars.

But since Volkswagen VOWG_p.DE admitted in 2015 to cheating on U.S. emissions tests, worries about vehicle pollution have left the entire auto industry under scrutiny. A particular concern is emissions by diesel cars of nitrogen oxide, which is blamed for causing respiratory diseases.

In the letter, Bienkowska told ministers she was concerned that the latest emissions violations at Audi NSUG.DE and Porsche PSHG_p.DE were discovered by prosecutors and not Germany's vehicle and transport authorities.

Bienkowska’s letter also called for all cars with excessively high levels of nitrogen oxide emissions to be taken of European roads, but said carmakers should act on a voluntary basis. The commissioner did raise the prospect of an EU testing agency if national regulators failed to spot more emissions-test cheats.

Munich, home to carmaker BMW BMWG.DE, has become the latest German city to consider banning some diesel vehicles. Environmental groups say diesel bans in cities can cut nitrogen oxide emissions and force automakers to design cleaner vehicles.

Experts who have seen the letter to ministers say the commissioner appeared to be bowing to carmakers’ demands.

“Her letter contained some important statements that we believe show the industry’s lobbyists have scored a big win,” Bernstein analyst Max Warburton said in a report.

“They have likely argued that castigating or banning diesel would harm the industry’s earnings and employees, harm efforts to reduce carbon dioxide and harm owners of current vehicles.”