Transport for London (TfL) is now charging drivers of older, more polluting vehicles to enter the congestion zone area of central London in hopes to reduce pollution levels.
It is estimated that about 40,000 vehicles will be affected every day.
Mayor of London Sadiq Khan said it was "important we make progress" in tackling the capital's toxic air.
Most vehicles which are not compliant with the new rules will have to pay £12.50 for entering the area each day, in addition to the congestion charge.
A vehicle's compliance can be checked using TfL's online checker but broadly speaking, those which are non-compliant are:
- motorbikes that do not meet Euro 3 standards (pre-2007 vehicles)
- petrol cars and vans that do not meet Euro 4 standards (vehicles pre-2006)
- diesel cars and vans that do not meet Euro 6 standards (vehicles pre-2015)
- Buses, coaches and lorries will need to meet or exceed the Euro 6 standards or pay £100 a day
Those who do not pay the charge will face a fine of £160, although a first offence may result in only a warning letter.
The ULEZ is set to be expanded to cover the Greater London area (the space between the North and South Circular roads but not inclusive of either) in October 2021.
Mayor Khan said London's air pollution was a "public health emergency" and it was the "poorest Londoners that suffer the worst quality air".
TfL estimates the initial scheme will lead to a reduction in toxic emissions from road transport by about 45% in two years.