Claimant law firms seek appeal in Diesel emissions legal battle
A group of car manufacturing giants may be forced back to the High Court to defend allegations that they installed devices to curb diesel emissions tests.
In the case nicknamed ‘Dieselgate’, the High Court handed down a judgment in July ruling in favour of the carmakers, but claimants, over a million motorists, a law firm revealed on Monday, are seeking permission to appeal the legal ruling.
The trial, which went ahead in October 2025, brought claims against 20 leading carmakers, including Mercedes, Ford, Renault, and Nissan, alleging they installed software in the cars’ emissions control systems from 2009 that reduced the effectiveness of nitrogen oxide (NOx) emissions under test conditions.
However, when the vehicles were driven on the road, it is alleged they produced much higher levels of emissions such as nitrogen oxides and particulates, which pollute the environment and cause a higher risk to people’s health.
Of the 20 vehicles, only three were found to have contained the so-called prohibited defeat devices (PDD), manufactured by Mercedes and Peugeot-Citroën. Lady Justice Cockerill found the other 17 vehicles to contain devices that did not breach emissions regulations.
Cockerill said in her judgment that “in the majority of instances, the court found that the relevant strategy did not constitute a prohibited defeat device.”
“Not every calibration or emissions-control strategy amounts to a defeat device,” the judge said.
Claimant firms to challenge judge’s definition
On Monday, the claimants’ legal team, in the largest group claim ever brought in the UK, Leigh Day, said it plans to appeal the judgment and challenge the judge’s interpretation of the emissions regulation and the device.
Leigh Day and Pogust Goodhead are the joint lead law firms working alongside 21 other firms on behalf of the 1.6m vehicle owners.
“Our appeal will argue that the judge applied too narrow a definition of what constitutes a Prohibited Defeat Device and that some of the technical evidence around this issue should be looked at again,” Martyn Day, Leigh Day senior partner, said.
Pogust Goodhead’s lead partner Anna Varga said their clients “consider that several of the findings should be appealed, including the overly narrow interpretation of what constitutes a prohibited defeat device.”
In a previous statement, a spokesperson for Mercedes said: “We believe that the claims are without merit and will vigorously defend ourselves against them or any group action with the necessary legal means. In our opinion, the emission control software functionalities in question are justifiable from a technical and legal standpoint.”
Mercedes, Peugeot/Citroen [owned by Stellantis], Ford, Renault, and Nissan were contacted for comment.