Volkswagen has employed a range of “progressively more innovative” arguments to claim the software fitted to millions of its vehicles is not a defeat device designed to cheat emissions tests, the high court has heard.

Lawyers representing more than 90,000 UK customers are bringing the biggest class action of its kind in Britain against the carmaker over the dieselgate emissions scandal.

VW admitted in 2015 to having manipulated the systems in 11m vehicles worldwide to fool emissions tests. It pleaded guilty in the US two years ago to criminal charges and paid out $4.3bn (£3.3bn) in civil and criminal penalties. In Australia it settled a multimillion-dollar class action which involved an agreement to pay between $87m and $127m in compensation to customers.

In the face of the allegations, the VW CEO, Martin Winterkorn, apologised on behalf of the company, saying: “I personally am deeply sorry that we have broken the trust of our customers and the public. We do not and will not tolerate violations of any kind of our internal rules or of the law.”

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But in Europe VW is still denying the software in question was an illegal defeat device – despite German regulators having ruled in 2015 that the software was designed to cheat emissions tests.

Lawyers for the UK customers opened a class action against VW in the high court on Monday.

In 2015 the company admitted fitting the software to 1.2m of its vehicles in the UK, and rolled out a “fix” to make cars compliant with emissions laws.

But VW’s case in papers submitted to the court is that this software was not designed to breach the legal testing regime and was therefore not a defeat device.

Tom de la Mare QC, representing VW’s customers, said in written submission to the court: “VW have raised a range of progressively more inventive arguments as to why the software function is not a defeat device – arguments which have changed over time, and have been been deployed selectively before different courts and tribunals.”

He added EU emissions regulations prohibited the use of defeat devices to prevent cheating of the emissions limits by manufacturers. He said the software VW fitted was clearly designed to defeat the emissions tests and was clearly a prohibited defeat device.

De la Mare said the vehicles were “optimised to minimise the amount of pollutants” in emissions tests, meaning they operated in a “completely different way in the street to how it operated in the test”.

“It is difficult to think of a more obvious cheat than the one VW used,” he added.

Lawyers for VW customers say emissions tests are designed to protect the public from diesel emissions. “Nitrogen oxides are extremely dangerous air quality pollutants, particularly in urban areas,” said De la Mare, adding exposure to nitrogen dioxide carried with it significant risk to human health and citing statistics showing pollution was “killing approximately 1,000 people a day in Europe”.

The Department for Environment, Food and Rural Affairs estimates exposure to nitrogen dioxide has an effect on mortality equivalent to 23,500 deaths annually in the UK.

De la Mare said internal VW documents showed that the company had “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”.

De la Mare warned that if the court ruled in VW’s favour, it would leave the regulatory regime “toothless in the face of a major fraud of precisely the kind it was surely designed to prevent”.

Q&A What is Labour's 'green new deal' Show Hide Labour's 'green new deal' - or 'green industrial revolution' - proposes a massive programme of state investment to rapidly decarbonise the economy, creating hundreds of thousands of green jobs, and transforming the way people live - from upgrading the housing stock to revitalising public transport, tackling the UK’s air pollution crisis to moving towards a more sustainable agricultural model. Although the initial investment would be huge, advocates say that would be dwarfed by the cost of not tackling the escalating climate crisis and point to wide-ranging economic benefits in the medium term, positioning the UK at the forefront of the emerging green industries, which are expected to dominate in the coming years. Crucially, the green new deal would tie radical environmental action to a worker-led “just transition”, where the rapid move from a carbon-based economy to a sustainable system is led by – and benefits – ordinary people. Its advocates say jobs in carbon-intensive industries would be replaced by – among others – those in wind, solar, house building and transport infrastructure. Labour's manifesto put the green new deal at the centre of its 2019 election campaign. The idea has also been championed by leading Democratic presidential candidates in the US 2020 race. Internationally, its supporters say any such proposal must also recognise the west’s historic responsibility for the crisis - and its global nature - and support a just transition in the developing world through transfers of technology and finance, while welcoming migrants. Matthew Taylor

VW said in a statement: “Volkswagen Group maintains that there has never been a defeat device installed in any of its vehicles in the UK.

“The question the judge is deciding in this hearing is not whether the affected vehicles contained such a device, but whether the legal definition is met in certain circumstances. We will continue to defend robustly our position in the high court.”

Dieselgate became public in September 2015 when the US Environmental Protection Agency revealed how VW and Audi had violated the Clean Air Act, leading to investigations by government agencies around the world in the biggest scandal to hit the car industry in decades.

In January former Volkswagen CEO Martin Winterkorn and four others were charged in Germany with fraud in connection with the emissions scandal. Winterkorn has denied any personal involvement with the scandal, and the case is not expected to take place until next year.