The environmental groups demanded at a press conference on Monday that the German government put in place more stringent tests to make sure that cars are actually clean and sustainable.

The groups said that all vehicles should be tested again while in operation, and be analyzed by the German Environmental Agency, rather than the federal transport authority (KBA).

Those who violate limits should be punished and there should also be measurements for traffic, the groups added.

The proposals were presented by groups including the Union for the Environment and Nature Conservation Germany (BUND) and Greenpeace.

The demands were also written as an open letter to Chancellor Angela Merkel.

The call for more comprehensive testing comes after it was revealed that Volkswagen had used software to cheat pollution tests for diesel engines, with the company admitting that 11 million cars worldwide had been equipped with the software. 

Some of the company's vehicles with the software were found to emit 40 times the legal levels of certain pollutants.

Volkswagen has faced international scrutiny for the scandal and has taken a huge hit to its revenues, as well as to its reputation.

CEO Martin Winterkorn was forced to step down last month and Volkswagen has vowed to get to the bottom of the emissions software.

German prosecutors said on Friday that they had identified fewer than ten people suspected of orchestrating the emissions manipulation.