Lawyers warn that the case could eventually involve up to 1.8 million UK drivers across 14 brands, potentially making it the largest consumer class action in British legal history.

A decade after Volkswagen’s emissions scandal shook the global auto industry, five of the world’s largest carmakers—Mercedes-Benz, Ford, Renault, Nissan, and Peugeot/Citroën—are now facing a landmark High Court trial in London that could permanently alter drivers’ perception of “clean diesel.”

The long-running “dieselgate” class action, which opened this week, brings together around 850,000 motorists who claim they were misled about the environmental performance of their vehicles.

Lawyers say the case could eventually expand to include as many as 1.8 million UK drivers across 14 brands, potentially making it the largest consumer class action in British history.

At the heart of the case are allegations that the manufacturers installed “defeat devices”—software designed to detect when a vehicle was undergoing official emissions testing and temporarily reduce pollution output to meet EU standards.

Outside testing conditions, however, the vehicles allegedly reverted to normal performance, emitting nitrogen dioxide and particulate matter far above legal limits.

All five companies have denied wrongdoing. Mercedes-Benz said its systems were “legally and technically justified,” while Renault and Stellantis insisted their vehicles complied with all European emissions regulations when sold.

Ford dismissed the lawsuit as “without merit,” and Nissan reiterated its “commitment to regulatory compliance in every market.”

The manufacturers are expected to argue that the software in question was intended to protect engines from damage rather than manipulate test results—a technical distinction that could prove crucial to the case’s outcome.

The High Court proceedings, expected to run for three months, will focus on a small sample of diesel vehicles produced by the five companies.

A judgment is not expected until mid-2026.

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