The High Court will decide in a three-month hearing whether systems installed in Mercedes, Ford, Peugeot-Citroen, Renault and Nissan diesel vehicles were designed to cheat clean air laws.

The trial of the five lead defendants will set a precedent for other manufacturers, potentially paving the way for billions of pounds in compensation.

The claims were brought on behalf of 1.6 million motorists against 14 carmakers, including Jaguar Land Rover, Toyota, Vauxhall-Opel and BMW, among others.

Martyn Day, lawyer at Leigh Day representing the claimants, said the case could lead to "serious amounts of compensation".

"The most important thing for us is to persuade the judge that there are defeat devices in all of these cars," he told AFP outside court.

The dieselgate scandal first erupted in September 2015, when German automaker Volkswagen admitted to fitting millions of vehicles with software to make engines appear less polluting in regulatory tests than in real driving conditions.

It caused waves in the global car industry, ensnaring several other top carmakers and leading to legal action in multiple countries including France, South Korea and the United States.

- 'Protect our children's health' -

The court in London will examine evidence to determine whether major carmakers installed defeat devices in cars to reduce nitrogen oxide readings in order to cheat emissions tests.

The five lead defendants deny that their systems were designed to circumvent the tests.

London resident Rosamund Adoo Kissi-Debrah, whose nine-year-old daughter Ella died in 2013 from an asthma attack linked to air pollution, joined around twenty protestors outside the court Monday.

"The coroner said if it wasn't for the illegal levels of air pollution where we live, not only would she not have got asthma, she wouldn't have died on that fatal night," she told AFP.

She hopes for "an apology" from the manufacturers.

"There are still millions of diesel cars on our roads and making our children sick," said Jemima Hartshorn, a clean air campaigner at Mums for Lungs.

She said she hoped the trial would encourage the government to "get these cars off our roads to protect our children's health".

- 'Hold to account' -

Adam Kamenetzky, one of the claimants, said he felt "defrauded" after he bought a Mercedes SUV in 2018 on the belief that it was less polluting than other models.

"We live in a built-up neighbourhood in London where there are children with lungs that can be harmed immeasurably by the emissions that these cars are producing," he told AFP.

Kamenetzky said outside the court that he wants to "hold to account" the manufacturers.

But it will take some time for any possible compensation to reach claimants.

The trial beginning Monday must first determine whether carmakers are liable, before a separate compensation phase can follow next year.

German automaker Mercedes and US carmaker Ford both rejected the claims as having "no merit", while Japan's Nissan declined to comment.

French manufacturers Renault and Stellantis, parent of Peugeot and Citroen, both said the vehicles they sold were compliant with regulations at the time.

When the High Court in 2020 found Volkswagen had used defeat devices to cheat emissions tests, the German automaker settled out of court, paying pound193 million ($259 million) to 91,000 British motorists.

Overall, to date, Volkswagen has had to pay more than 32 billion euros ($37 billion) in penalties over the scandal, mostly in the United States.