Thomas de la Mare KC, for the motorists, said the case revealed a pattern of behaviour across the industry. "Each player in the industry basically took a conscious decision that customer convenience, which helped the industry sell more cars, was more important than preventing pollution," he said.

He added that the manufacturers had valued sales over compliance. "They have basically said the concern about making these cars sellable by removing these inconveniences is so strong, it is so valuable, that we would rather cheat than comply with the law."

In written evidence he cited a report by the Centre for Research on Energy and Clean Air, which found that excess nitrogen oxide from diesel engines was linked to 12,000 premature deaths and 98,000 cases of childhood asthma in Britain and Europe between 2009 and last year.

The case comes nine years after the "dieselgate" scandal, when Volkswagen admitted fitting defeat devices to millions of cars worldwide. VW later paid £26 billion in fines and compensation and agreed a £193 million settlement with British motorists in 2022.

Lawyers say the latest proceedings could have even greater financial consequences; there are 47 models from 20 brands under investigation. The five manufacturers were chosen to face trial first due to the scale of their alleged involvement. Cases against nine more carmakers are expected to follow depending on the outcome.

Defence teams rejected the allegations. Neil Moody KC, for Ford, said the claims were "scientifically illiterate" and "flawed on the facts and the law". He added: "The inference seems to be of some sort of industry-wide conspiracy. The proposition needs only to be stated for it to be seen to be implausible."

Alexander Antelme KC, representing Renault, also dismissed the accusations as "without merit and untenable".

Mercedes-Benz said: "We believe the claims are without merit and will vigorously defend ourselves against them."

Outside court, clean air campaigners gathered in support of the case. Among them was Rosamund Adoo-Kissi-Debrah, whose nine-year-old daughter, Ella, became the first person in Britain to have air pollution listed as a cause of death.

Holding a placard reading "Clean air is a human right", she said the trial was a vital step in holding the car industry to account.

Five carmakers in court accused of evading emissions tests