Audi's admission came after the German Transport Ministry said it was investigating suspicions that the carmaker had installed a new illegal defeat device in some tens of thousands of A6/A7-model vehicles worldwide, around half of them on German roads.

The German news weekly Spiegel said mass recalls were highly likely. According to the magazine, the cars were equipped with software that deliberately slowed down the use of a special pollution-cleaning fluid toward the end of its lifespan to avoid drivers having to refill the AdBlue liquid in between regular service updates.

But reducing the AdBlue function also drastically lowers its effectiveness in neutralizing the engine's harmful nitrogen oxides, making the diesel cars far more polluting.

Audi insisted it had reported the irregularities to the authorities itself as soon as the issue was detected in routine testing and "immediately halted deliveries" of the affected models.

It said customers would be notified and offered a software update.

Not a first

It's not the first time that German carmakers have been accused of AdBlue tampering, with Daimler and Volkswagen (Audi's parent company) both facing the threat of recalls over similar accusations in February.

The suspected AdBlue scam differs from the scandal that sparked Volkswagen's Dieselgate crisis in 2015, when the auto manufacturer admitted to installing defeat devices in some 11 million cars worldwide.

The devices were able to detect when a given vehicle was undergoing pollution tests in the lab and reduce emissions accordingly.

Audi said in a statement that cars in the United States were not affected by its latest woes.

hg/jd (AFP, Reuters)