Mercedes-Benz could face another wave of recalls after German transport authority KBA deemed inadmissible defeat devices installed in some of its vehicles, in a letter leaked by a whistleblower to environmental NGO Deutsche Umwelthilfe (DUH), Reuters reported.

The newswire further said that in a letter published on DUH's website, the KBA referred to three devices found in cars produced under the Euro 6 standard, the most recent and stringent limits for harmful tailpipe emissions from petrol or diesel-powered vehicles.

Mercedes-Benz was obliged to take remedial measures or face an order to remove the cars from the road, it said, based on a ruling by the European Court of Justice that deemed such devices illegal in certain driving conditions, Reuters said.

The KBA was not immediately available for comment.

In a statement, Mercedes-Benz said it was co-operating fully with the KBA and that it believed it had already developed the technology required to tackle the issue via software updates, Reuters said.

The carmaker had received an extension beyond the originally requested date of July 27 to identify the number of cars affected and address the issue, the carmaker said, without giving the new deadline, Reuters reported.