Volkswagen "cheated" European emissions rules designed "to save lives" by installing unlawful "defeat devices" in diesel cars, the High Court has heard.
Tens of thousands of UK motorists who bought VW, Audi, Seat and Skoda diesel cars are taking legal action in the aftermath of the "dieselgate" scandal.
The claimants' QC Tom de la Mare said: "It is difficult to think of a more obvious cheat than the one VW used."
Volkswagen has said it will "defend robustly its position".
In 2015, VW admitted 11 million cars worldwide - including 1.2 million in the UK - had software that reduced readings of emissions in tests. However, the UK hearing, expected to last two weeks, centres on whether that software constitutes a "defeat device" under EU regulations.
In his opening remarks, Mr de la Mare told the court that VW engines were "optimised to minimise the amount of pollutants" in emissions tests, meaning the vehicles operated in a "completely different way in the street to how it operated in the test".
He added: "It is difficult to think of a more obvious cheat than the one VW used." Mr de la Mare said European emissions standards were designed "to save lives", adding that "the most up-to-date evidence" showed that pollution was "killing approximately 1,000 people a day in Europe".
He said internal VW documents showed that the company has "long known that the software was unlawful and indefensible", pointing to one document in which a VW employee said the vehicles would "flunk" emissions tests without the software.
He submitted that the documents showed a "clear acceptance that the software was the only basis on which they were meeting the emissions limits".