Dieselgate explained: Five carmakers go on trial over emissions cheat claims

Five major carmakers - Mercedes, Ford, Peugeot/Citroën, Renault and Nissan - are facing trial at the High Court in one of the largest class actions in UK history over alleged emissions cheating.

The lawsuit, which kicked off on Monday, centres on accusations that the companies installed "defeat devices" to produce artificially low readings of nitrogen oxide emissions. A defeat device is software that alters engine performance during testing. Around 220,000 motorists are directly involved in this phase of the case, with claims potentially affecting up to 1.6 million UK car owners. Depending on the outcome, nine other manufacturers could face similar proceedings.

The scandal, dubbed "dieselgate", first emerged in 2015 when US regulators found Volkswagen had fitted these devices to bypass emissions standards.

The UK High Court ruled in 2020 that Volkswagen had breached EU rules, leading the firm to settle for £193million with British drivers outside of court, as part of payouts exceeding €32billion (around £27.8billion) worldwide.

Lawyers for motorists argue the deat devices misled customers about the environmental impact of their cars, which they claim still emit harmful pollution on the road.

They allege this deception has exposed millions to dangerous levels of nitrogen oxides, which could have potential health consequences.

All five manufacturers have denied wrongdoing. Mercedes insists its testing mechanisms were technically and legally justified. Renault and Stellantis, the owner of Peugeot and Citroën, have said vehicles met regulations at the time of sale.

Ford has called the claims "without merit", while Nissan has asserted full compliance with market rules.

The current trial will determine whether the systems breach emissions law, and the judgement is expected to come in summer 2026.

If the court rules against the carmakers, a follow-up trial in autumn 2026 will decide compensation levels.

Lawyers have said the verdict could expose "one of the most egregious breaches of corporate trust in modern times".