Motorists may have to pay more to drive in central London as a new ultra-low emission zone (Ulez) takes effect from today.
Drivers of diesel cars roughly over four years old and petrol cars about 13 years old must pay £12.50 to enter the centre of the capital.
Charges apply at all times and are on top of the already existing congestion charge, which is £11.50 between 7am and 6pm on weekdays.
All vehicle types apart from black taxis are liable for the Ulez charge unless they meet certain emissions standards or exemptions.
Drivers can check whether their vehicle is liable for a charge by entering its registration on the Transport for London website.
The Ulez will be extended to the whole of inner London within the North and South Circular roads from October 2021.
London mayor Sadiq Khan said the scheme is being introduced because thousands of Londoners are dying early every year as a result of toxic air, with an increased risk of cancer, asthma, dementia and stroke.
Mr Khan commissioned a study by two universities which found that poor air quality leads to about 1,000 London hospital admissions for asthma and serious lung conditions every year.
He said: "As someone who developed adult-onset asthma over the last few years, I know from personal experience that London's toxic air is damaging people's health.
"This study is a stark reminder that air pollution disproportionately affects the most vulnerable Londoners and I'm doing everything in my power to protect children, the elderly and those with respiratory conditions from our filthy air."
The Ulez was announced by former London mayor Boris Johnson, but his successor Mr Khan brought its start date forward and decided on the 2021 extension.
There has been concern that poorer motorists, small businesses and charities will be unfairly hit by the charge as they are less able to afford to upgrade their vehicles.
London director of business organisation the CBI, Eddie Curzon, described the Ulez as a "really positive step" but warned that "smaller firms can struggle to afford the switch to low-emission vehicles".
"To make a success of the Ulez, it is crucial that City Hall works with firms to help them take advantage of new technologies and support them, where required, to accelerate the take-up of low emission vehicles," he said.
City Hall said Transport for London is running a scrappage scheme to help the smallest businesses and charities switch to cleaner vehicles.
It also noted that people in London's most deprived areas are more likely to suffer from poor air quality and are least likely to own a car.