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".
VW's barrister, Michael Fordham QC, argued in written submissions that the claimants had misunderstood the legal definition of a defeat device.
In a statement before the hearing, a VW spokeswoman said: "Volkswagen Group continues to defend robustly its position in the High Court in London.
"It remains Volkswagen Group's case that the claimants did not suffer any loss at all and that the affected vehicles did not contain a prohibited defeat device."
Volkswagen has faced a flurry of legal action worldwide, and has been forced to pay out more than €30bn (£26bn) in fines, recall costs and civil settlements. The carmaker's current and former senior employees are facing criminal charges in Germany.
The English litigation was filed back in 2016, but has now reached what the claimants' lawyers have called "a decisive court battle".
Gareth Pope, head of group litigation at Slater and Gordon, which represents more than 70,000 of almost 90,000 claimants, said before the start of the hearing: "This trial will establish once and for all whether VW installed prohibited 'defeat devices' in affected vehicles and is a significant milestone in our clients' attempts to hold VW accountable in the UK.
"This is a decisive point for VW. For years, the carmaker has deceived its customers, marketing cars as complying with emissions standards while all the time knowing they were emitting many times more than the allowed level of toxic pollutants, perpetrating an environmental and health scandal.
"VW has had plenty of opportunity to come clean, make amends and move on from this highly damaging episode.
"But instead it's chosen to spend millions of pounds denying the claims our clients have been forced to bring against it rather than paying that to their own customers in compensation."
One of the claimants, Brian Levine - who bought two affected Volkswagens - told the Press Association: "VW's tactics have been to delay and prevaricate - anything but face a day when it would have to explain what this software did. Well, that day has finally come.
"More than four years after the emissions scandal broke, the tens of thousands of customers will be able to hold VW to account in a British court of law and expose its efforts to cheat us."
Buying a car is one of the big financial decisions we make. We pay a hefty price for a brand we trust and expect the specifications to be as promised.
But VW argues that the drivers claiming they were fooled by a defeat device will not qualify for compensation whatever the merits of their case, because they haven't suffered a loss.
And VW denies having a defeat device, anyway, despite findings against the carmaker in other countries.
Waiting for a result from this case will feel like being trapped in the mother of all traffic jams. Lawyers involved are expecting the whole process could take two to three years.