Nearly 700,000 drivers in London could be subject to a daily £12.50 fee when the Ultra Low Emission Zone (Ulez) expands this summer.
The expansion of the zone to cover the whole of London will be introduced on August 29.
This will mean around 691,559 licensed cars will be liable for the charge, according to figures from the Driver and Vehicle Licensing Agency obtained by the RAC.
This figure rises to 851,065 when counting vehicles such as vans and lorries.
However, the RAC the number of drivers affected once the zone expands is "likely to be far higher" when taking into account vehicles entering from bordering counties such as Kent, Hertfordshire and Essex.
The motoring group has warned that the expansion will have a "massive financial impact on motorists and businesses", as many will be forced to switch or scrap their vehicles.
Transport for London (TfL) has recently said nine out of 10 cars driving in outer London meet the Ulez standards, so will not be liable for the charge.
Most diesel cars registered before September 2015 and petrol cars registered prior to January 2006 could be made to pay the charge.
As of November 2022, TfL estimated that the number of compliant vehicles within the whole of Greater London stood at 90.5%
However, outer London currently has the lowest proportion of ULEZ-compliant vehicles, with just 85% estimated to be meeting the standards as of January 2023.
The reason for the expansion of the zone, which currently covers the area within London’s North and South Circular roads, is to improve air quality in the city.
Nicholas Lyes, head of roads policy at the RAC said that cleaning up London's air should "undoubtedly be a priority" however, the number of vehicles that don't meet the requirements cannot be ignored.
He said: “We desperately need more coordination between the Mayor and the Government to help small businesses, tradespeople, NHS staff and carers who have no choice but to drive into the expanded ULEZ for work purposes from outside Greater London.
"Consideration should also be given to those who work at night when public transport is greatly reduced in the outer boroughs.
“Changing to a compliant vehicle at such short notice simply won’t be something many will be able to afford, especially during a cost-of-living crisis and at a time when second-hand car prices are so high."
At the moment, TfL is running a scrappage scheme for vehicles that do not comply with the Ulez standards.
The scheme offers payments of up to £2,000 for those on low incomes or those who are disabled to scrap their non-compliant car, and up to £9,500 for charities, sole traders and small businesses scrapping or retrofitting vans and minibuses.
Nicholas added: "We need more creativity from London’s Mayor and his team to help people out as the current scrappage scheme is akin to using a plughole to drain an Olympic-sized swimming pool.
"It’s simply not big enough for the scale of the job.
“We believe he should consider introducing an additional one-year sunset period for certain key workers or a scheme where TfL partners with a leasing company to provide discounted, ULEZ-compliant vehicles to smaller businesses or traders, enabling them to avoid daily charges while still reducing roadside emissions.”
