The transport secretary has said drivers considering buying diesel cars should take a "long, hard think".
Chris Grayling made the remarks to the Daily Mail, which said the government was considering a scrappage scheme for older diesel cars.
Concerns over nitrogen dioxide (NO2) emissions from diesel vehicles have been raised in recent years.
The Department for Transport said Mr Grayling was not telling people to stop buying diesel vehicles.
It declined to comment on reports of a new scrappage scheme.
According to statistics from the Department for Environment, Food and Rural Affairs (Defra), NO2 is responsible for about 23,500 deaths in the UK each year.
Concern over emissions increased when it emerged that 1.2 million Volkswagen diesel vehicles in Britain had been fitted with software to help cheat emissions tests.
Mr Grayling told the Mail: "People should take a long, hard think about what they need, about where they're going to be driving, and should make best endeavours to buy the least polluting vehicle they can.
"I don't think diesel is going to disappear but someone who is buying a car to drive around a busy city may think about buying a low-emission vehicle rather than a diesel."
Greenpeace clean air campaigner Areeba Hamid said: "It's a bit confusing. He's saying 'have a long and hard think about diesel' but in the same breath he's saying [diesel cars] won't disappear."
She said the government should deliver a strong message to the car industry and consumers by changing the taxation structure on diesel cars in the next Budget.
The Society of Motor Manufacturers and Traders (SMMT) chief executive Mike Hawes said: "The biggest air quality gains will come by encouraging the uptake of the latest low emission vehicles, regardless of fuel type."
Steve Fowler from Auto Express magazine said the government should not "penalise" those who "really have no alternative" to using diesel.
"As much as battery cars, hybrid cars are improving, they're never going to be the greatest things for really long journeys and for things like towing, so diesel - for the moment - will always have a place," he said.
"And people living in rural areas - this is where the one size fits all thing doesn't work."
When guest editing BBC Radio 4's Today programme last December, Britain's chief medical officer, Professor Dame Sally Davies, said diesel cars should "steadily be phased out" in order to reduce deaths from air pollution.
London is one of the worst affected areas in the UK for air pollution, and the city's mayor Sadiq Khan has asked the government to adopt a £515m diesel scrappage scheme to help reduce emissions in the capital.
Mr Khan has also said a £10 "toxicity charge" - which will target the most polluting older vehicles in the capital - will come into force on 23 October.
A spokesman for the Department of Transport said the government was helping to tackle air quality by providing a further £290m to support electric vehicles.
The spokesman added: "We will update our air quality plans later this year to further improve the nation's air quality."
The Labour government ran a £300m scrappage scheme for both diesel and petrol cars between 2009 and 2010.