LONDON (Reuters) - Tens of thousands of British drivers on Monday accused Volkswagen VOWG_p.DE of fitting devices to cheat clean air laws at the start of the country's biggest class action lawsuit brought to tackle "dieselgate".

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VW was caught using illegal software to cheat pollution tests in 2015, triggering a global backlash against diesel and numerous court cases around the world that have so far cost the German company 30 billion euros ($33 billion)

Volkswagen has said about 11 million cars worldwide - and 1.2 million in Britain - were fitted with software that cheated diesel emissions tests designed to limit noxious car fumes and carbon dioxide (CO2) pollution.

A hearing at the High Court is set to last for two weeks.

“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,” Gareth Pope, head of group litigation at law firm Slater and Gordon, said. Slater and Gordon is representing more than 70,000 VW customers.

The High Court will determine two issues of law. Firstly whether the software installed in vehicles was a “defeat device” under EU regulations and whether the High Court is bound by the German Regulator’s finding that the software was a “defeat device.”

Volkswagen said the answer to both cases was no.

“Volkswagen Group continues to defend robustly its position in the High Court in London,” it said in a statement. “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.

“The hearing will not affect any questions of liability or loss.”

VW agreed to pay up to $25 billion in the United States to settle claims from owners, environmental regulators, states and dealers. It offered to buy back 500,000 polluting U.S. vehicles.

But the company has not reached a similar deal in Europe and has instead offered a software update. It argues it has not broken any English law.