Car manufacturers are facing a major legal challenge in British court cases, ten years after the 'dieselgate' scandal

Some of the world's biggest carmakers face a crucial trial at London's High Court on Monday, with lawyers representing 1.6 million claimants accusing them of cheating diesel emissions tests, a decade after Volkswagen's 'dieselgate' scandal.

In one of the largest mass lawsuits in English legal history, owners of diesel vehicles from Mercedes-Benz, Ford, Nissan, Renault and Stellantis' Peugeot and Citroen brands allege that the companies used unlawful 'defeat devices'.

These devices detected it during vehicle testing and ensured emissions were within legal limits, but failed to do so while the cars were in transit, plaintiffs' attorneys say.

However, the manufacturers say the claims are fundamentally flawed and reject any similarity to the scandal that broke out in 2015 and cost Volkswagen billions of euros in fines and compensation.

Mercedes-Benz said its emissions control systems were legally and technically justified.

The trial will focus on a small sample of diesel vehicles produced by the five manufacturers, which are being sued by nearly 850,000 plaintiffs, to determine whether they used prohibited defeat devices. Any damages the court says should be paid will be decided at a new trial next year.

The court's ruling will also be binding on hundreds of thousands of similar claims against other manufacturers, including Stellantis-owned Vauxhall/Opel and BMW.

Martyn Day, one of the plaintiffs' lawyers from the firm Leigh Day, said the allegations, if proven, would demonstrate “one of the most egregious breaches of corporate trust in modern times”.

When VW admitted to using defeat devices in emissions tests, it led to the automaker having to pay more than 32 billion euros ($37 billion) in vehicle refits, fines and legal fees, while former chief executive Martin Winterkorn faced criminal charges, although his trial was suspended this month on health grounds.

It is not the first time that London's High Court has been asked to rule on defeat devices, having ruled against VW in 2020. VW settled these claims without any admission of liability in 2022.

The current group of claims, against a total of 14 manufacturers, is much larger than the VW case, with plaintiffs' lawyers previously valuing the lawsuit as a whole at around 6 billion pounds ($7.97 billion).

Automakers are facing lawsuits around the world, including in the Netherlands, where a court ruled in July that diesel cars sold by Stellantis brands Opel, Peugeot-Citroen and DS contained defeat devices, a ruling Stellantis said was incorrect.

Manufacturers and suppliers have also paid fines and reached settlements in the United States and elsewhere to resolve investigations into diesel vehicle emissions.