Used diesel vehicle values have declined by more than 10% since 2017 as demand shifts towards petrol models.
New data from Motorway.co.uk shows that used petrol values increased by 10% during the same period.
The average diesel value in Q1 2017 was £14,327, dropping to £13,605 by Q4 2017, and £12,849 a year later – a fall of £1,478 (or 10.3%) for the average used diesel.
Meanwhile, the average petrol price went up +10% from £8,912 in Q1 2017 to £9,807 by Q4 2018, an increase of £895 over the same period of the study.
Most major car brands including Ford, Mercedes and Volkswagen saw diesel price drops, with the biggest falls seen with Land Rover, BMW, Vauxhall and Audi.
The average price of a used BMW diesel car has slumped by 16.1% since the start of 2017, falling almost £3,000 from £17,269 in Q1 2017 to £14,441 in Q4 2018.
This compares with average prices of BMW petrol variants which have remained steady (falling just 0.6%) over the same period.
While the average price of a diesel Audi has fallen 14.3% since the beginning of 2017, compared to a 8.4% increase in price for the petrol variants.
The average price of a Land Rover - almost all of which are diesel-powered - was down more than £6,000 from £30,493 to £24,399 between Q1 2017 and Q4 2018 (a fall of 20%).
Motorway.co.uk analysed two years of valuations for more than 130,000 used cars valued on its website, which aggregates live offers from popular online car buying websites and used car dealers.
Alex Buttle, director of Motorway.co.uk, said: “It’s been a tough couple of years for diesel owners. Having been penalised by the Government and ripped apart by the press, diesel prices slumped in 2017.
"Although we saw early signs of resilience in the first half of 2018, any recovery has been undone recently by worsening consumer sentiment towards diesel.
"This followed the announcement of wider, more punitive emission zones in London and more rigorous emissions testing for all cars later in the year.
“Increasingly, people looking to buy a used car are deciding on fuel type before choosing what make and model they want to purchase.
"More people are opting for petrol and diesels are suffering. Despite newer Euro-6 diesel engines now matching petrol equivalents for efficiency, consumer sentiment has simply moved away from diesel – possibly forever.”