Research suggests a number of popular family models – including some Volkswagen, Ford and Nissan vehicles – are among those unable to run on the E10 eco-fuel.
The news comes after the Department of Transport (DfT) launched a consultation on its proposal to roll-out the more environmentally-friendly E10 in larger filling stations across the country in 2020.
The Government hopes the move towards E10, already on sale throughout France and Germany, will help the UK meet its emissions targets.
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Regular unleaded petrol on sale in the UK contains up to 5% renewable bioethanol to help reduce carbon emissions, but biofuel E10 involves increasing this proportion to 10%.
Environmental agency ePURE suggests switching to E10 fuel could see greenhouse gas emissions from UK petrol vehicles cut by around 6%.
But the study by the RAC Foundation, estimates there will be 634,309 motors on the road incompatible with E10 when the Government rolls out the eco-fuel.
The most common of these cars is projected to be Volkswagen Golf, with an estimated 28,066 incompatible models likely to still be in use in two years’ time.
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Steve Gooding, director of the RAC Foundation, played down the number of incompatible cars and stressed that that vast majority of UK cars will be able to run on E10.
He said: “This analysis shows that even in a couple of years' time there will still be hundreds of thousands of cars on our roads that are incompatible with the new fuel.
“Whilst some of the cars incompatible with E10 fuel will be historic models, many will be old but serviceable everyday run-arounds that people on a tight travel budget rely on to get about.
“The good news is both that the vast majority of cars on our roads are able to run on E10 and that Transport Secretary Chris Grayling has recognised the need to protect the users of those older vehicles which are not E10 compatible.”
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