Interest in diesels has soared because prices are being slashed.
Measures to crack down on polluting diesels were announced by the Government last week.
Drivers were warned they could be charged to drive in city centres and even banned from some roads.
Owners are also braced for a potential tax hike in the autumn budget.
Environment Secretary Michael Gove, who unveiled the measures to improve Britain’s air quality, said harmful emissions had to be cut and climate change tackled.
Diesels are being targeted because their emissions are more toxic than previously suspected.
Ministers have pledged to end sales of diesel and petrol cars and vans by 2040 to be replaced by electric vehicles.
However, the price of diesel models has been lowered because of the announcement.
It is also driving down the price of second-hand versions, leading to fears the move could backfire and put more of the cars on the road.
Alex Rose, head of trading at independent online car-buying site carwow, said: “Diesel is starting to regain popularity with UK drivers.
“We’ve seen a gradual increase in interest in diesel models in July, accounting for 43 per cent of sales.”
The Government has been forced to draw up the clean-up plan to limit harmful nitrogen dioxide pollution, after it lost a long-running legal battle over European Union emission limits.