TfL says about 70,000 fewer cars enter the charge zone each day
Transport for London (TfL) has blamed road works for a rise in congestion since the introduction of the congestion charge.
It said traffic - the volume of cars entering the central London £8 road toll zone - was down 21% since the charge came into force in 2003.
But congestion - time taken to make journeys - was back to 2002 levels, a TfL report has shown.
The study blamed reduced capacity due to utility companies' road works.
TfL's sixth Annual Impacts Monitoring Report showed 70,000 fewer cars enter the original zone each day compared to pre-charging levels, and 30,000 (14%) fewer cars enter the western extension since it was introduced in 2007.
OTHER REPORT FINDINGS
The Congestion Charge generated provisional net revenues of £137m in 2007/08
There has been a 6% increase in bus passengers during charging hours in the western extension since 2007
There has been a 12% increase in cycle journeys into the western extension since 2007
But the report said congestion has to return to levels experienced before the charge was introduced.
Between 2003 and 2006 congestion fell by between 20% and 30%, compared to pre-charging levels.
But in 2006 and 2007 this reduction stood at an average of 8%, which has been blamed on a sharp increase in road works in the capital.
TfL claims that, since April 2007, the number of road work notices served to local councils across London has averaged between 12,000 and 14,000 each month.
TfL's Malcolm Murray-Clark said: "Without the congestion charge the traffic problems in London would be much worse.
The congestion charge has proved successful in cutting traffic coming into London but on its own has not resolved the problem of congestion
"However, as a result of other interventions such as utility and construction works, the reduction in road space has had a detrimental impact on congestion levels and is slowing traffic down."
But critics of the scheme said blaming road works was not the answer.
"Over the last five years or so, we've lost a lot of road capacity, particularly in central London, with roads being closed, roads being narrowed, pavements being widened, bus lanes being introduced and traffic lights being re-phased," said Edmund King, from the Automobile Association.
"It's a combination of those measures which actually means that congestion's worse and unfortunately motorists who are paying £8 a day to come into London are not really getting value for money."
London Mayor Boris Johnson called the scheme a "blunt instrument" which has not helped combat congestion.
"It has proved successful in cutting traffic coming into London but on its own has not resolved the problem of congestion," he said.
He has proposed re-phasing traffic signals and working with utility companies to minimise the impact of road works.