Traffic moving through the congestion zone during the rush hour travels at an average of just 9.3mph, compared to around 9.9mph in 2003.
Statistics compiled by Transport for London (TfL) show that charging did improve traffic flow initially.
However, congestion relief has slowed and journey times are on the rise.
Though the volume of traffic entering the zone has fallen by 21%, the improvement in congestion has fallen dramatically since 2003, from 30% to only 8%.
Off peak and evening rush hour travel speeds have also decreased after an initial improvement.
TfL blamed road maintenance for the growing congestion, pointing out that the number of street works doubled last year, taking up road space and constricting traffic flow.
A TfL spokesperson defended congestion charging: “Without it, London would have ground to a halt. We have made real progress, and the benefits in central London remain clear.”
Meanwhile, complaints that luxury car drivers were using a loophole to avoid the capital’s congestion charge have prompted TfL to review all cars that are registered as private hire vehicles.
All private hire cars registered with TfL are exempt from the charge.
A spokesman said it is now checking its list of exempt vehicles. “We have already been in touch with many of the owners of luxury vehicles registered as private hire vehicles and are satisfied that the vast majority of these are being used for legitimate private hire purposes,” she said.
“Only a relatively small number of luxury vehicles are licensed as private hire vehicles.”