London’s Ultra Low Emission Zone, or Ulez, is set to be extended to the capital's outer boroughs next month, forcing hundreds of thousands of drivers to pay daily fees.
The divisive plan is part of mayor Sadiq Khan's bid to clean up the city's toxic air.
But critics argue it is an unfair tax on motorists who are already grappling with a cost-of-living crisis.
The anti-pollution scheme is expected to cost some London drivers £4,500 a year.
On August 29, the zone will be expanded from its current borders of the North and South Circular roads to encompass outer London boroughs.
The changes will mean about 700,000 people with older car models, classed as high-polluting, will have to pay a daily fee to drive in London.
What is Ulez?
Launched in 2019, the Ulez is the world's first scheme that requires vehicles to comply with anti-pollution measures 24 hours a day, seven days a week.
Separate from the congestion charge, which is aimed at reducing traffic, Ulez is designed to cut air pollution in the capital by discouraging the use of high-emission vehicles through imposing a daily fee.
It aims to improve the health of Londoners by reducing the amount of particulate matter and nitrous oxide in the air.
How serious is air pollution?
Government health officials believe particulate matter and nitrous oxide lead to the deaths of between 28,000 and 36,000 people every year and estimate a £1.6 billion ($2.05 billion) cost to the NHS between 2017 and 2025, with vehicle exhausts being the main source of those gases.
In 2020, Ella Kissi-Debrah became the first person in the world to have air pollution cited as a cause of death.
She died in 2013 aged nine, after suffering from an asthma attack brought on by ingesting traffic fumes near her home in south-east London.
To which vehicles does Ulez apply?
All cars, motorcycles, vans, minibuses and other specialist vehicles weighing up to 3.5 tonnes.
Generally, petrol cars registered after 2005 and diesel cars after 2015 meet the emissions standards.
Cars older than this are charged £12.50 a day, with a penalty for non-payment of up to £180.
How do I know if my vehicle is Ulez-compliant?
Drivers can check whether their vehicle meets the emissions standards on Transport for London’s website by entering their registration number.
Automatic number-plate-recognition cameras are set up along streets within the zone that check the registration with DVLA records to determine the vehicle’s age and therefore its compliance.
Whose idea was Ulez?
The scheme was first approved in 2015 when Boris Johnson was London mayor but was introduced four years later under Mr Khan’s stewardship.
Mr Khan has since been a strong supporter of Ulez, advocating its expansion towards the outer boroughs.
Why is it expanding?
At first, Ulez applied to central London but in 2021 it grew to border the North and South Circulars as part of a pandemic bailout agreement between Transport for London (TfL) and the government.
Mr Khan said he wanted to expand the zone further to encompass the outer London boroughs to lower the air pollution in those areas.
Opponents believe the mayor is using it as a way to make money for TfL.
Why are people opposed?
Critics of the Ulez expansion say the scheme disproportionately affects poorer people who need to drive for work and discourages sole traders from outside London from taking work in the city.
A scrappage scheme is in place for people on benefits with older cars to receive up to £2,000 or a mix of cash and public transport passes, but critics say the money is not enough.
Could Ulez be ditched?
The Conservative-run outer London boroughs of Bexley, Bromley, Harrow and Hillingdon along with Surrey County Council have taken legal action against the mayor of London in the High Court, saying he lacks the legal power to order the scheme’s extension.
They are expecting a judgment on that case before the end of this month, which could delay the extension, making it a prominent campaign issue in next year’s mayoral and general elections and Labour may choose to reconsider backing it.