Shares in the three biggest German car makers have fallen after a report claimed they colluded for years over diesel emissions technology.
BMW shares were down 2.6%, Daimler was down 3.7% and Volkswagen 2.6%. Shares also fell on Friday after Der Spiegel published its findings online.
Spiegel reported that employees from Volkswagen, Audi, BMW, Mercedes and Porsche had met often since the 1990s and had agreed to limit the size of the tanks holding a urea solution used to reduce diesel emissions of harmful nitrogen oxide.
The smaller tanks reduced costs and freed up space in the vehicles, the magazine said.
BMW issued a statement denying that its urea tanks were too small to provide adequate exhaust treatment and said its vehicles' emissions met legal requirements.
Officials at the European Union's executive Commission said in a statement that they and the German competition authority have received information on the matter, which is currently being assessed by the Commission.
The Spiegel report follows announcements last week by Daimler that it is recalling three million Mercedes-Benz diesels to improve their emissions performance through an update of engine control software, and by Volkswagen's luxury Audi brand that it was doing the same with 850,000 vehicles.
Daimler also said it would speed up the deployment of new engines. BMW is offering software updates on 350,000 of its older diesels.
The steps are a way to head off calls for banning diesel vehicles from some German cities where air pollution levels exceed limits.
The German government has summoned local officials and car executives to a "diesel summit" on August 2 to find ways to reduce emissions and ensure that diesel technology has a future.
The industry is a major employer, and diesels are considered one way to meeting goals for lower emissions of greenhouse gases blamed for global warming. Diesels emit less carbon dioxide, a major greenhouse gas, but emit more nitrogen oxide, a pollutant that harms people's health.
Diesels came under increased scrutiny after Volkswagen admitted to using illegal software that in the US detected when vehicles were on test stands, and turned emission controls on so that the cars passed the emissions test.
The controls were turned off in everyday driving, improving mileage and performance.
Volkswagen has agreed to more than 20 billion dollars in US civil and criminal fines and settlements, and eight executives have been charged.
Separately, five German car-makers - Daimler's Mercedes-Benz, Opel and Volkswagen and its subsidiaries Audi and Porsche - last year agreed to recall a total of 630,000 diesel vehicles in Europe after it was found that real-world emissions often exceeded EU emissions standards.
In those cases, engine control software turned off emission controls at certain temperatures to avoid engine damage.
That was legal, but German regulators have questioned whether the use of the exemption was always justified.