New figures have blown a "massive hole" in the mayor's argument for increasing the congestion charge to £8, according to the London Assembly's Lib Dems.
The road toll will rise in July to £8
Mayor Ken Livingstone said the £3 rise, which comes into force on 3 July, was necessary to cut traffic levels.
The Lib Dems claim congestion has been declining over the past nine months and is down 2% on last year.
Transport for London (TfL) says the rise is needed because the "deterrent effect is going to degrade over time".
The TfL spokesman said that while the congestion charge has clearly helped reduce traffic levels, figures will not remain at this level.
Lib Dem transport spokesperson, Lynne Featherstone, said: "These figures show that there is no increase in traffic levels and so no need for any treatment.
"The mayor should be shouting from the rafters just how successful the existing congestion charge scheme has been in reducing traffic.
"This 60% price hike increase flies in the face of common sense.
"It has nothing to do with reducing congestion but everything to do with shoring up TfL's rocky finances."