Motorists seeking respite from the heat by sitting in stationary cars with the air conditioning running are being warned they could be fined under emissions laws.
The RAC said remaining in a vehicle with its engine running while stationary contributes to air pollution.
Standard penalties for such an offence are £40 if issued by a police officer, or £80 from a civil enforcement officer, with both amounts halved for prompt payment.
However, some local authorities use their powers to impose larger fines.
In London, Islington Council, for instance, said its enforcement officers issue £110 fines to people who refuse to switch off an idling engine after being asked.
RAC senior policy officer Rod Dennis said: “Air conditioning is a gamechanger in modern vehicles, making otherwise uncomfortable journeys by road a genuinely pleasant experience.
“We even advise those driving vehicles without air-con to consider delaying any non-essential trips this week until the heatwave passes.
“But while we’re all desperate to stay cool, just sitting in the car with the engine and air-conditioning running and going nowhere isn’t advisable.
“Due to the air pollution this causes, you risk a fine from a police or civil enforcement officer.”
In 2019, the Conservative government announced plans to introduce higher fines for drivers who idle, but this never took place.
Lord Chris Grayling, then-transport secretary, said at the time that combating idling would be “an easy way to drive down dangerously high levels of pollution”.
