Luxury carmaker Porsche is to challenge plans to increase London's congestion charge to £25-a-day for some vehicles.
The firm said it intended to ask for a judicial review into the price changes, which come into force in October.
Porsche says the new rules, which affect the highest polluting vehicles, are "disproportionate" and will not decrease emissions in central London.
Transport for London (TfL) predicts up to 22,000 cars will no longer come into the zone when the new fee comes in.
Under the plans, which will be introduced on 27 October, cars with the lowest carbon dioxide (CO2) emissions will get a 100% discount on the charge.
Most drivers will still pay the £8 road toll for entering the zone which covers parts of central and west London.
Porsche will write to the mayor's office informing him of their intention to seek a judicial review.
If it does not receive a satisfactory response, it will apply to the courts for an order to quash the new charge.
The mayor's advisor on climate change Mark Watts said "No-one needs to drive these really big polluting cars in central London.
The £25 charge will apply to vehicles emitting more than 225 grams of CO2/km (g/km), as well as those registered before March 2001 which have engines larger than 3,000cc
Those vehicles getting a 100% discount will emit less than 120g/km.
Of cars currently being driven in the congestion charging zone, 17% would be liable for the £25 charge and just 2% for the total discount
"It's the 'polluter pays' principle - you will be allowed to carry on driving a big polluting car in central London, but what the new charge will mean is you have to pay for the cost of the pollution that you inflict upon everyone else."
The mayor has said: "There will be losers but overall we will all gain."
But Andy Goss, managing director of Porsche GB, said: "A massive congestion charge increase is quite simply unjust.
"Thousands of car owners will be hit by a disproportionate tax which clearly will have a very limited effect on CO2 emissions."
Porsche vehicles which would be affected by the new charge include the Cayenne, which can be bought with a 4.8-litre engine at a cost of around £50,000, and the 911 model, which has an engine size of up to 3.8 litres and has a price tag of up to £80,000.
Friends of the Earth (FoE) director Tony Juniper said: "Cutting carbon dioxide emissions is now an urgent priority and all sectors of the economy will need to play a part in that process.
"Porsche has founded its business on the promotion and supply of highly polluting vehicles."
President of the AA Edmund King said: "We are concerned that the Mayor has changed his scheme from a congestion charge to an emissions charge almost overnight.
"Larger families with some people carriers or estate cars will also be hit."
Transport for London (TfL) estimates about 33,000 vehicles will now fall into the £25 charge sector to drive into London each day.
It predicts about two-thirds of these will no longer come into the charge zone once the new fee is introduced.
The congestion charge was initially introduced in February 2003 and covered just central London, with the daily charge set at £5.
Since then the charge has gone up to £8 a day and a western extension, incorporating such areas as Kensington and Chelsea, has been added.
According to TfL, there are 70,000 fewer vehicles in the charging zone, every day, in the five years since the fee was introduced.