New cars sold in Britain last year were more harmful to the environment than those in 2016 because of the “demonisation” of diesel.

Data from the Society of Motor Manufacturers and Traders (SMMT) revealed that average CO2 emissions from cars sold last year were higher than in 2016, reversing a near 20-year decline.

The increase – to 121.04 grams of CO2 per kilometre from 120.1g/km – is being blamed by the trade group on the backlash against diesel vehicles, which pump out less CO2 than petrol vehicles.

Motorists are abandoning cars powered by diesel, the SMMT’s preliminary annual figures show, with a 17pc annual plunge in diesel sales in the wake of the Volkswagen scandal and confusion over the government’s policies towards the fuel.

Ministers want to improve air quality by reducing harmful nitrous oxides, which diesels generally produce more of than petrol cars.