Car manufacturers have welcomed a high court verdict that rejected most of the allegations in a “dieselgate” claim brought on behalf of 1.6 million UK owners of polluting cars.
In her judgment, Lady Justice Cockerill said that “in the majority of instances, the court found that the relevant strategy did not constitute a prohibited defeat device” – software that enables the engine to behave differently in tests.
However, she said that technology and strategies used in some Mercedes and Peugeot-Citroën cars could constitute defeat devices.
Lawyers for the claimants said they were considering appealing, saying the judgment created a significant divergence between the legal position in Great Britain and much of Europe.
The claimants had argued that manufacturers had engineered vehicles with technology that would reduce nitrogen oxide (NOx) emissions in tests compared with normal driving conditions. The manufacturers denied using prohibited defeat devices.
The case against the manufacturers, the largest group action trial in English legal history, was heard in the high court over 15 weeks between October 2025 and March 2026.
It focused on 20 vehicles sold by five manufacturers – Mercedes, Ford, Renault, Nissan and Peugeot/Citroën – from 2009, but the judgment also binds other manufacturers.
The judge said: “Not every calibration or emissions-control strategy amounts to a defeat device.”
Cockerill said proving intention to rig a test was necessary, adding: “It was not enough for the claimants simply to establish that the challenged strategies reduced the effectiveness of emissions-control systems outside the relevant testing conditions.”
The judgment found that: “Testing, however, is difficult to interpret and none of the approaches to isolating the effect of a particular calibration on NOx was entirely satisfactory.”
Her ruling also said that “if an alternative approach to the meaning of ‘defeat device’ were taken a larger number of defeat devices would be established, including devices in each of the lead manufacturers cars”.
Mercedes-Benz said the court had ruled “very largely in favour” of the manufacturer but said it was considering an appeal over the vehicle deemed non-compliant. The functionality was removed from vehicles in 2015.
Stellantis, the owner of Peugeot-Citroën, said it was considering an appeal over the upheld allegations.
Nissan said it had always maintained that the technologies in its vehicles did not constitute defeat devices. Renault said its cars were “designed, engineered, and manufactured in accordance with all applicable regulatory requirements”.
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All four welcomed the ruling. Ford was contacted for comment.
This first UK legal verdict came more than a decade after it was first discovered that Volkswagen car engines were designed to operate differently under test conditions and masked the true level of NOx pollution from the vehicles.
The case was brought by more than 20 law firms, led by Leigh Day and Pogust Goodhead. Leigh Day said the ruling was “disappointing” and meant that defeat devices that would be likely to be found to be illegal across the EU were allowed in Great Britain.
Senior partner Martyn Day said: “The high court has found that despite their denials, other manufacturers also used VW-style defeat devices. The judge has also found that many other defeat devices are lawful because she surprisingly disagreed with the established EU case law on what a defeat device is.
“We are considering with our clients whether to apply for permission to appeal.”
Pogust Goodhead’s Anna Varga said the judgment “does not bring this litigation to an end. The court has found that certain manufacturers installed unlawful defeat devices, but it also adopted a significantly narrower interpretation of the law than that applied elsewhere in Europe.”
The campaign group Mums for Lungs said it was a “setback”, but its director, Jemima Hartshorn, said: “Whatever the courts have found, it does not change the scientific consensus – that these diesel cars are toxic. It’s important that the two car companies found guilty today act swiftly to remove their vehicles and compensate for their actions.”