Luxury carmaker Porsche has launched an online petition to back its case for a judicial review against London mayor Ken Livingstone's plan to tax gas guzzlers in the city centre.

On its website, Porsche urge people who consider the proposed charge unfair to write to newspapers and the mayor, letting him know their opposition.

Porsche also hit back at the mayor by claiming that annually his c-charge plan would save the equivalent of just four minutes of CO2 emissions at Heathrow Airport.

With Transport for London's own figures, the manufacturer claimed the savings expected in an entire year from the emissions charge would be equal to the emissions produced from Heathrow airport in a period of between four minutes and, at most, four hours.

Andy Goss, managing director of Porsche Cars GB, said: "Under closer examination these figures show the negligible environmental benefits of this tax.

"Not only is this new charge an unfair tax on motorists and families, it is a tax that will do nothing to reduce emissions or congestion in London."

Mr Livingstone confirmed he had received Porsche's letter setting out the basis of their proposed legal challenge to the CO2 charge on the most CO2 emitting vehicles in the congestion charging zone.

A spokesman said the mayor would contest the action vigorously.

"Porsche has a vested interest in seeking to prevent London government from exercising its powers to improve the environment. Surveys show a big majority of Londoners support a higher congestion charge for the most CO2 emitting vehicles," he said.