Motorists are shunning diesel cars, as they turn to buying vehicles that are much more eco-friendly, industry figures suggest.
The Society of Motor Manufacturers and Traders (SMMT) said that 78,778 diesel cars were sold in January, a drop of 4.3% on the same month last year.
Over the same period sales of electric cars and other alternatively-fuelled vehicles (AFVs) jumped by 19.9%.
For the first time, AFVs now account for more than 4% of the market.
The figures show that 7,270 AFVs – including hybrids – were sold in January, gaining a 4.2% market share.
Sales of diesel cars have been falling for several months, following publicity about pollution and health issues. And revelations that Volkswagen – and possible other manufacturers – used engine software to distort emissions data during tests have not helped diesel’s reputation.
In December 2016, sales of diesel cars were down by 6.8% on the same month a year earlier.
Analysis: John Moylan, Industry Correspondent
Are the headlines over diesel cars finally hitting sales? We bought a record number of cars in 2016.
But the growth in diesel sales lagged the wider market. That growth, of 0.6%, hid the fact that the move against diesel had accelerated throughout the year.
Diesel car sales have fallen in 7 of the past 8 months, compared with the previous year.
Today’s figures for January seem to confirm the trend.
They show that while a year ago diesel sales were outnumbering those of petrol cars, by January 2017 that situation had reversed.
And while the market share of diesel and petrol cars was neck and neck in 2015, in January diesel’s market share fell to 45.1% compared to petrol’s 50.7%.
Several big cities around the world have said they want to ban diesel cars within 10 years, because of the pollution they cause.
A group of doctors has called on the Mayor of London to ban them in the capital, while Westminster will hit some diesel drivers with extra parking charges from April.
The government is also thought to be planning a diesel scrappage scheme, to encourage motorists to ditch cars which are the heaviest polluters.
However, the SMMT figures show that overall sales of vehicles in the UK last month were at their highest January level in 12 years.
In total 174,564 cars were sold, a rise of 2.9% on January 2016.
Motorists who buy electric or hybrid petrol/electric cars still enjoy a grant from the government, worth up to 35% – or £4,500 – of the total value.