A major lawsuit against five of the world's biggest carmakers began at London's High Court on Monday, with lawyers accusing them of manipulating diesel emissions tests.

The case is the latest chapter in the "dieselgate" saga, in which carmakers have been accused of using software to reduce harmful emissions under test conditions.

It has been 10 years since the dieselgate scandal first emerged in the United States, when regulators revealed the Volkswagen Group had installed "defeat devices" in millions of diesel cars so they could pass lab emissions tests while emitting far higher emissions on the road.

That discovery triggered global investigations, recalls, and settlements costing Volkswagen tens of billions of US dollars in fines and compensation.

Given the scale of the new case, the court has selected five manufacturers — Mercedes, Ford, Peugeot/Citroen, Renault, and Nissan — to be the lead defendants and to be tried first. All of the companies deny the allegations.

The outcome of the three-month trial could open the door to similar claims against nine other carmakers. Lawyers said the lawsuit could ultimately involve 1.6 million car owners and is the largest group action case to be brought in English and Welsh legal history, the BBC reported.

Lawyers representing the owners of diesel vehicles made by the companies allege they used the unlawful defeat devices to cheat clean air laws. The manufacturers deny the allegations.

Martyn Day, from the legal firm Leigh Day, one of 22 entities representing drivers, said: "A decade after the dieselgate scandal first came to light, 1.6 million UK motorists now get their chance to establish at trial whether their vehicles contained technology designed to cheat emissions tests."

He added that, if the allegations against the carmakers are upheld, it "would demonstrate one of the most egregious breaches of corporate trust in modern times".

"It would also mean that people across the UK have been breathing in far more harmful emissions from these vehicles than they were told about, potentially putting the health of millions at risk."

The trial will test a sample of diesel models from the five manufacturers, covering nearly 850,000 claimants, with any damages decided next year.

The ruling will also be binding for hundreds of thousands of similar claims against other carmakers, including Stellantis-owned Vauxhall/Opel and BMW, Reuters reported.

London's High Court has ruled on defeat devices before, finding against Volkswagen in 2020. But the current group of claims is far bigger than in the Volkswagen case, spanning 14 manufacturers, and is valued at about 6 billion pounds ($8 billion). Worldwide, automakers face similar actions and have been paying fines or reaching settlements in the United States and elsewhere.