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

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Germany’s cartel authority declined to comment on the report, which sent car stocks tumbling.

Volkswagen VOWG_p.DE, Daimler and BMW shares were down 3.9 percent, 2.7 percent and 2.8 percent respectively, underperforming Germany's blue-chip DAX index which was down 1.9 percent by 1401 GMT.

The Stoxx 600 autos index was down 3.1 percent.

“This new chapter in the diesel saga needs to be taken seriously,” Evercore ISI analyst Arndt Ellinghorst said in a note. “Our conclusion is that there might be a risk of several hundred millions or even low billions.”

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.

A spokesman for Volkswagen VOWG_p.DE, which owns the Porsche and Audi brands, declined to comment.

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, Spiegel said.

The manufacturers 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 which owns the Mercedes-Benz brand, declined to comment.

BMW was not available for immediate comment.