The new charge will be introduced in October
The mayor of London has said he will contest legal action by Porsche against plans to increase the congestion charge for "gas guzzlers" in central London.
The luxury car maker is seeking a judicial review over plans to increase the charge to £25-a-day for the most polluting vehicles.
Ken Livingstone's office said it had now received a letter setting out Porsche's legal challenge.
Porsche said a huge increase in the congestion charge was "simply unjust".
Speaking last week Andy Goss, managing director of Porsche GB, said: "Thousands of car owners will be hit by a disproportionate tax which clearly will have a very limited effect on CO2 emissions."
On Friday, Porsche launched an online petition to back its case for a judicial review.
In opposition to this, environmental campaign groups Friends of the Earth, Greenpeace and the Alliance Against Urban 4x4s have also launched a joint online petition.
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.
A spokesman for the mayor said: "Porsche's claim that it is illegal for the mayor to introduce this policy is wrong.
"The Greater London Authority Act 1999 gives the mayor the power to do this.
"Porsche has a vested interest in seeking to prevent London government from exercising its powers to improve the environment.
"The mayor will contest this action vigorously."
If Porsche does not receive a satisfactory response to the letter, it will apply to the courts for an order to quash the new charge.