Demand for new diesel cars fell 25% last month, new figures show.
Around 51,000 new diesel cars were registered in April compared with 68,000 during the same month in 2017, the Society of Motor Manufacturers and
Traders (SMMT) said.
The market share for diesels fell from 45% to 31%.
There has been growing concern about the impact of diesel car emissions on air quality and uncertainty about what taxes and restrictions will be introduced in relation to the vehicles.
The overall new market rose by 10.4% year-on-year in April, bucking the trend for the year.
The SMMT said this was partly due to changes to Vehicle Excise Duty (VED) in April 2017, which reduced sales for that month as buyers brought purchases forward to March 2017.
Registrations of petrol cars were up 39% last month, while demand for alternatively fuelled vehicles such as hybrids and pure electrics increased by 49% to take a market share of 6%.
Mike Hawes, the organisation's chief executive, said it is important not to look at one month in isolation as April's year-on-year increase is "not unexpected".
He went on: "While the continuing growth in demand for plug-in and hybrid cars is positive news, the market share of these vehicles remains low and will do little to offset damaging declines elsewhere.
"Consumers need certainty about future policies towards different fuel types, including diesel, and a compelling package of incentives to deliver long-term confidence in the newest technologies."
Richard Jones, managing director of motor finance provider Black Horse, which is part of Lloyds Banking Group, said the "ongoing confusion" around fuel choice remains "a major concern".
He added: "The public policy debate on diesel has led many customers to the mistaken view that the biggest issue to be addressed in reducing emissions is the removal of diesel cars from the UK's roads.
"Rather, the most important issue is actually the more general removal of older, more polluting cars - whether petrol or diesel."