Current proposals will see ULEZ expanded to cover the whole city from August this year. The zone was first introduced in April 2019 and charges non-compliant vehicles – mostly diesel cars that are more than six years old and petrol cars that are more than 15 years old – £12.50 for each day they are in the zone.
Transport for London said that more than four in five vehicles in outer London, which would be affected by the expanded zone, already meet ULEZ standards.
The London boroughs of Bexley, Bromley, Harrow and Hillingdon and Surrey County Council have formed a coalition to mount a legal challenge against the action.
It includes allegations that the scheme fails to comply with statutory requirements and that the proposed scrappage scheme, which is designed to help people affected by the scheme to update their vehicles, was not consulted upon.
Councillor Ian Edwards, leader of Hillingdon Council, said: "Our position has remained unchanged from when TfL's plans were first mooted - ULEZ is the wrong solution in outer London as it will have negligible or nil impact on air quality but will cause significant social and economic harm to our residents.
"We shared this view in our response to the TfL consultation last summer and we've said it since when the plans were confirmed in November. Now, we'll say it in the courts.
"We believe Sadiq Khan's decision to impose this scheme on outer London boroughs is unlawful - his spending nearly £260m of public money without any cost benefit analysis. Hillingdon, and the other coalition local authorities wouldn't dream of making decisions in this fashion.
"The predominant effect of ULEZ expansion will be to financially cripple already struggling households, further isolate the elderly and harm our local economy with negligible or no improvement to air quality. Investment in improved transport links - on a par with those in areas within the existing ULEZ - is the better way to reduce car use in Hillingdon."
TfL argues that the government has consistently failed to provide it with enough additional funding since its income sources were hit by reduced travel following the pandemic. In 2021, it warned that it was considering closing more than 100 bus routes as it could not afford to keep them operating.
While there have been significant improvements in London's air quality in recent years, 99 per cent of the city's residents still live in areas that exceed World Health Organization limits for air pollution, specifically PM2.5 particulate matter, which is emitted by vehicles, among other sources.
A spokesman for the Mayor of London said: "While we're aware of media speculation that an application for a judicial review has been made by four boroughs and Surrey County Council, neither the GLA (Greater London Authority) nor TfL have been served with their claim.
"We will be defending any challenge to this vital scheme. Around 4,000 Londoners die prematurely every year due to air pollution. The Mayor is determined to protect the lives of Londoners who are growing up with stunted lungs and more at risk of heart disease, cancer and dementia due to our toxic air.
"The Mayor urges the councils involved to abandon this costly and unnecessary legal challenge and instead focus on the health of those they represent."
