Drivers of older, more polluting vehicles now have to pay almost twice as much to drive in central London.
Mayor Sadiq Khan's £10 T-Charge, which mainly applies to diesel and petrol vehicles registered before 2006, has come into force.
It covers the same area as the existing congestion charge zone, bumping up the cost to £21.50 for those affected.
Opponents said the scheme would "disproportionately penalise London's poorest drivers".
The measure is the latest attempt by Mr Khan to improve air quality in the capital and, according to the mayor's office, will affect 34,000 motorists a month.
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From Monday 23 October, there is a £10 daily fee for those who drive more polluting vehicles in the congestion charging zone, on top of the existing £11.50 congestion charge.
Vehicles that do not comply with the Euro IV exhaust standard must pay the £10 charge.
The standard defines emissions limits for cars, vans, buses, coaches and lorries. Most vehicles registered before 2006 are likely to exceed these limits.
The zone will operate between 07:00 and 18:00, Monday to Friday.
Speaking on the Today programme, Mr Khan said: "We've got a health crisis in London caused by the poor quality air.
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