Demand for new diesel cars plummeted by a fifth last month amid rising concerns over air pollution.
Around 81,500 new diesel cars were registered in May, down 20% on the same month last year, according to the Society of Motor Manufacturers and Traders (SMMT).
This represented a fall in market share from 50% to 43.7%.
Air pollution is linked to an estimated 40,000 early deaths a year and 37 out of 43 areas across the UK are exceeding legal European Union limits for key pollutant nitrogen dioxide, much of which comes from diesel engines.
Volkswagen Group admitted in September 2015 that almost half a million of its diesel vehicles in the US were fitted with software to switch engines to a cleaner mode when they were being tested for emissions.
The manufacturer said 11 million of its vehicles were affected worldwide - including almost 1.2 million in the UK.
Jim Holder, editorial director of What Car? magazine, said he was surprised by "the scale of the drop-off" in demand for new diesel cars.
He told the Press Association: "T here is no clear government strategy on diesels: There has been a wave of negative headlines around the fuel but at the same time the taxation system continues to incentivise them.
"These mixed messages do nothing for consumer confidence, nor does the blanket reference to all diesels being dirty.
"There have been significant steps in reducing emissions, particularly with the latest Euro 6 regulation compliant units that are on sale now."
Last month the Government published long-awaited plans to improve air quality, including a "targeted" scrappage scheme to take the most-polluting vehicles off the road.
Clean air zones are being established in several towns and cities across England, which could involve charges for the oldest and dirtiest vehicles.
The most polluting vehicles will be forced to pay up to £24 a day to drive in central London from 2019 under plans unveiled by Mayor Sadiq Khan.
Mr Holder predicted that sales of diesel cars will continue to suffer while there is uncertainty over future policy.
" Until there is clarity over where future tax incentives will lie, and what city centre charges will or won't be imposed, diesel sales will remain under pressure," he said.
"Consumers have no clear indication of the financial or environmental impact of their decision. That is bad for both legislators and car buyers."
In May registrations of new petrol cars rose by 0.4%, while demand for alternatively fuelled cars increased by 46.7%.
The overall new car market fell by 8.5%, the second consecutive month of decline.
Many buyers brought orders forward to March ahead of new vehicle excise duty (VED) rates coming into force which mean the vast majority of new cars now incur a larger fee.
SMMT chief executive Mike Hawes said: "We expected demand in the new car market to remain negative in May due to the pull-forward to March - which was an all-time record month - resulting from VED reform.
"Added to this, the General Election was always likely to give many pause for thought and affect purchasing patterns in the short term."