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FRANKFURT (Reuters) - European antitrust authorities plan to fine carmakers Daimler BMW and VW up to 1 billion euros ($1.1 bln) each for colluding on reducing the effectiveness of exhaust filtering systems, German weekly Der Spiegel said.

The German carmakers collaborated to reduce the size of so-called Adblue tanks which help to reduce harmful nitrogen oxide emissions, and agreed not to include filters on gasoline engine vehicles to reduce fine particulate matter, Der Spiegel said.

BMW declined to comment. Daimler said it is cooperating with the European authorities and has applied for the status of a cooperating witness in the proceedings. Volkswagen said it is cooperating with authorities.

Strategic cooperation among German carmakers is not unusual, but companies found guilty of breaching EU cartel rules face fines of as much as 10 percent of their global revenues.

The European Commission declined to comment.