FRANKFURT (Reuters) - Germany's carmakers VW (VOWG_p.DE), BMW (BMWG.DE), Audi, Porsche may have colluded to fix the prices of diesel emissions treatment systems using industry committees, German magazine Der Spiegel reported on Friday.

Around 200 employees sitting in 60 industry committees discussed vehicle development, brakes, petrol and diesel engines, clutches and transmissions as well as exhaust treatment systems, Der Spiegel reported, citing a letter sent to cartel authorities.

Volkswagen admitted to possible anti-competitive behavior in a letter it sent to cartel authorities on July 4, Der Spiegel said.

The carmakers discussed their choice of suppliers and the price of components. Since 2006, the carmakers have also discussed the cost of AdBlue, an exhaust emissions treatment system for diesel engines, it said.

They discussed details such as the sizing of tanks for diesel emissions treatment fluid and they agreed to use smaller rather than larger ones, Der Spiegel said.

Daimler (DAIGn.DE) which owns the Mercedes-Benz brand, declined to comment.

A spokesman for Volkswagen (VOWG_p.DE), which owns the Porsche and Audi brands, also declined to comment. BMW was not available for immediate comment.

(Reporting by Edward Taylor; editing by Jason Neely)