Mercedes-Benz is recalling hundreds of thousands of cars in the UK for a software update to reduce their nitrogen oxide (NOx) emissions.
Almost every new diesel bought over the past six years is affected by the move, which the company said was voluntary and would involve owners being contacted to be offered the upgrade.
German owner Daimler said it was taking the action across Europe covering three million vehicles in total.
It refused to give a country-by-country breakdown but said one million were sold in Germany. It is believed the UK numbers are still being finalised.
The UK is one of its biggest customers in the EU – with 170,000 Mercs sold in the country last year alone.
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The vehicles affected have diesel engines meeting the Euro 5 standard – implemented in 2011 – and the existing Euro 6 rules which further limited the output of NOx and particulates – blamed for thousands of deaths each year.
Sky News was seeking clarification from the company on whether the upgrade would have any effect on a vehicle’s performance such as its acceleration.
Daimler said it was taking the action, at a cost of £190m, to “reassure” customers as many manufacturers move to shun diesel technology in the wake of the Volkswagen emissions scandal of 2015.
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VW admitted then it had fitted so-called defeat devices to diesel cars sold in the US to cheat emission testing regimes.
It has since been forced to pay billions in fines and compensation in the US.
The company also remains the subject of criminal and regulatory scrutiny in Europe and is facing civil action by drivers in the UK who are demanding compensation, claiming they were also duped. VW denies it broke EU law.
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The controversy prompted VW and its rivals to concentrate their development power on electric and hybrid technology as campaigners demand the brakes are applied to diesel technology on health grounds.
Dr Dieter Zetsche, the head of Mercedes-Benz Cars, said: “The public debate about diesel engines is creating uncertainty – especially for our customers.
“We have therefore decided on additional measures to reassure drivers of diesel cars and to strengthen confidence in diesel technology.
“We are convinced that diesel engines will continue to be a fixed element of the drive-system mix, not least due to their low CO2 emissions.”