To help improve air quality Transport for London have introduced the Ultra Low Emission Zone (ULEZ) which now operates 24 hours a day, 7 days a week, every day of the year within the same area of central London as the Congestion Charge.
Most vehicles, including cars and vans, need to meet the ULEZ emissions standards or their drivers must pay a daily charge starting from £12.50 to drive within the zone. The ULEZ will be extended to cover most of London in October 2021.
https://tfl.gov.uk/modes/driving/ultra-low-emission-zone