Expanding the zone would avoid the need to introduce two other levies – a £2-a-day “clean air charge” or a boundary charge of up to £5.50 for drivers entering greater London.

The charge varies depending on the type of vehicle. Most petrol and diesel-powered cars, vans and motorcycles will need to pay a £12.50 daily charge. Vehicles weighing more than 3.5 tonnes are exempt.

Mr Khan’s proposals come after Ella Adoo-Kissi-Debrah became the first person in the UK to have air pollution listed as a cause of death.

Her mother, Rosamund Adoo-Kissi-Debra, said: “People will be encouraged to get rid of their dirty vehicles, and children like Ella will breathe more easily and have a better quality of life.”

However, experts warned that the proposed ULEZ expansion could backfire. Andy Marchant from satnav maker TomTom said: “When [electric vehicles] become the norm, there won’t be any emissions-based restrictions preventing them from entering cities.

“There’s the possibility that this will lead to an increase in inner-city congestion, even if it generates zero-tailpipe emissions.”

Nevertheless, environmental campaigners welcomed the plans. Zack Polanski, a Green Party Member of the London Assembly, said: “The mayor says we only have a small window of opportunity to get this right – and that’s right. This is a good interim step but there’s no time to waste as we need road users charging to clean up London’s air.