Theresa May has indicated she will not punish drivers of old diesel cars if the Government cracks down on the motors to protect the environment.
Polluting cars will be forced to pay up to £24 a day to drive in central London from 2019 under plans unveiled by Mayor Sadiq Khan.
The Government has to publish updated clean air plans by April 24, after the courts ruled existing plans to meet EU-mandated air quality limits, which are being broken across many areas of the country, were not sufficient.
Drivers were encouraged to switch away from petrol under Tony Blair's government and Mrs May said that would be taken "into account" in future plans.
Mrs May said: "In relation to the issue of diesel cars, obviously we will be producing a new air quality plan, we've been required to do that by the courts.
"Decisions will be taken when we produce that plan - obviously we will take final decisions as to what we do. But I'm very conscious of the fact that past governments have encouraged people to buy diesel cars and we need to take that into account when we're looking at what we do in the future."
In June last year then-transport secretary Patrick McLoughlin said it had been a mistake for former Labour chancellor Gordon Brown to slash taxes on diesel.
Mr Brown reduced duty on low-sulphur fuel in 2001, which contributed to an increase in annual diesel car registrations from 3.45 million to 8.2 million.
A Government report published in April 2016 showed that diesel cars being sold in the UK emit an average of six times more nitrogen oxide in real-world driving than the legal limit used in official tests.
All but the newest diesel cars will face a £12.50 charge to drive in the planned ultra low emissions zone (Ulez) under the London mayor's plans.
Diesel cars that are more than four years old in 2019 and petrol cars that are more than 13 years old will face the charge 24 hours a day, year-round, in a bid to cut air pollution.
With the congestion charge during weekday hours, the total fee for the most polluting cars to drive through the heart of London would be £24.
Buses, coaches and HGVs that do not meet the emissions standards will have to pay £100. The Ulez will apply to all types of vehicle except black taxis.