Mercedes, Ford, Renault, Nissan and Peugeot/Citroën face group action in which damages could exceed £6bn

Car manufacturers decided they would rather cheat to prioritise "customer convenience" and sell cars than comply with the law on deadly pollutants, the first trial day of the largest group action in English legal history has been told.

More than a decade after the original "dieselgate" scandal broke, lawyers representing 1.6 million diesel car owners in the UK argue that manufacturers deliberately installed software to rig emissions tests.

They allege the "prohibited defeat devices" could detect when the cars were under test conditions and ensure that harmful NOx emissions were kept within legal limits, duping regulators and drivers.

Should the claim be upheld, estimated damages could exceed £6bn. The three-month hearing that opened at London's high court on Monday will focus on vehicles sold by five manufacturers - Mercedes, Ford, Renault, Nissan and Peugeot/Citroën - from 2009. In "real world" conditions, when driven on the road, lawyers argue, the cars produced much higher levels of emissions.

The judgment on the five lead defendants will also bind other manufacturers including Jaguar Land Rover, Vauxhall/Opel, Volkswagen/Porsche, BMW, FCA/Suzuki, Volvo, Hyundai-Kia, Toyota and Mazda, whose cases are not being heard to reduce the case time and costs.

Opening the case for the claimants, Thomas de la Mare KC said: "Each player in the industry basically took a conscious decision that customer convenience, which helped the industry sell more cars, was more important than reducing pollutants causing death."

In written submissions, the claimants quoted a Centre for Research on Energy and Clean Air report that found excess NOx emissions had caused 124,000 premature deaths in the UK and Europe between 2009 and 2024.

De la Mare told the high court levels of emissions could have easily been reduced had vehicles had larger tanks for AdBlue - an additive that cuts NOx in diesel vehicles - and asked customers to refill them. Instead, he said: "They have basically said the concern about making these cars sellable by removing these inconveniences is so strong ... that we would rather cheat than comply with the law."

The lead barrister also told the judge, Mrs Justice Cockerill, that manufacturers were trying to argue that numerous prior dieselgate rulings abroad, including in the US and European court of justice, did not apply to the UK. He said: "Our position is broadly that we will rely on foreign regulatory decisions as a matter of fact, rather than an opinion on law."

He said that the defendants' argument meant that "mainland GB becomes a sort of defeat device Brexit Island ... But Northern Ireland is applying case law that you're being asked to put in the bin."

Internal training and briefing documents from Ford in 2010 on defeat devices showed, he said, that: "The manufacturers could and did understand the law exactly as we do now."

Lawyers for the defendants will make opening statements later this week. The car manufacturers deny having used prohibited defeat devices.

In written submissions, lawyers for Renault, said the case was "riddled with errors and misunderstandings, especially about the design and operation of the vehicles' emission control systems", while Ford's said it was "scientifically illiterate".

Mercedes said the claimants case was "obsessively focused" on NOx pollution rather than other emissions, and did not "acknowledge the complexity" of diesel engineering. Lawyers for Nissan said the case's central legal premise was "an untenably broad reading of defeat device".

The main trial, establishing the evidence, is expected to conclude before Christmas, with a break before legal arguments are heard in March 2026. A judgment is expected next summer, with a further trial for possible compensation following.

Adam Kamenetzky, from south London, a lead claimant among the 1.6 million owners, told the Guardian outside court: "For the first time, certainly in the UK if not in any jurisdiction, the evidence is actually being made public.

"If these allegations are borne out, consumers were deceived at the point of sale, and there needs to be action taken to prevent this kind of deception being built into cars."

Carmakers chose to cheat to sell cars rather than comply with emissions law, 'dieselgate' trial told