London Mayor Ken Livingstone's plan to turn the capital's congestion zone into a low emissions zone has been put on ice, Autocar has learned. Three weeks ago we reported on Transport for London's plan to introduce a sliding scale tariff in place of the current £8-a-day congestion charge. As part of the scheme, cars emitting less than 120g/km of carbon dioxide, such as the Smart Fortwo, Citroen C1 and Toyota Prius, will be able to enter the city centre for free, while those emitting more than 225g/km of CO2 – sports cars, luxury saloons, SUVs and a great many others – will be liable for a whacking £25-a-day sting.Autocar understands that TfL has appointed consultants to advise on how best to implement the new system, and how markedly it might affect CO2 emissions in the city centre. According to various sources, among them The Guardian and the BBC, the Mayor's plan was to put this new carbon emissions-based charge in place in spring 2008, in time for the London Mayoral election. However, it now appears that the system won't be fully practicable until 2010, by which time the Congestion Charge's automatic number plate recognition camera network will have been replaced by an 'electronic tag and beacon' system, which should be much cheaper to operate than the current scheme, and easier to use for London's drivers.In a direct response to questioning from Autocar on his Guardian blog, Livingstone advised that "the proposal to charge cars entering the congestion charge zone according to their CO2 emissions has not been scrapped. Following consultation, we intend to introduce discounts for the least polluting vehicles in 2008, and to introduce surcharges for the most polluting vehicles by 2010."