Motorists who did not pay London's congestion charge because they drive green vehicles may lose their discount.
Transport for London has scrapped the Greener Vehicle Discount, which applied to vehicles emitting 100g/km CO2 or less.
It is replacing it with the Ultra Low Emission Discount with provides a concession to vehicles that emit 75g/km of CO2 or less.
The penalty for not paying the charge has also risen from £120 to £130.
A spokesman for Transport for London said there were about 20,000 motorists currently eligible for the Greener Vehicle Discount.
They will be granted a three-year "sunset period" before they are made to pay the full congestion charge, he said.
The congestion charge for cars entering the central London zone currently stands at £10 - or £9 for drivers who are registered to pay via an automated payment service.
Professor Stephen Glaister from the RAC Foundation said: "Over time vehicles have got much more efficient and produced less CO2, so it makes sense to reduce the threshold and only allow the very lowest emitters to have the concession."
Matthew Pencharz, the mayor of London's advisor on the environment, said: "These changes are in line with the mayor's aim to improve air quality in London by reducing emissions from private vehicles and promoting the further development of low emission vehicles.
"We want to encourage the continued development of these technologies, while also protecting the benefits to traffic flow in the centre of London that the charge provides."