The firm which operates the congestion charge is failing to meet performance targets set by the Mayor of London, the Liberal Democrats have said.
TfL said there was a "steady" improvement in performance
They claim Transport for London (TfL) figures show that out of 39 key targets, 21% were not met by Capita - costing them £100,000 a month in fines.
But TfL said there had been a "steady and marked" improvement in performance.
The Lib Dem's said "serious questions" must be asked if Ken Livingstone extended Capita's contract.
Lynne Featherstone of the Lib Dem's said: "While Capita has improved its service, it is still failing to meet its obligations in many areas.
'Very good scores'
"The mayor should adapt a more haste, less speed approach to awarding new contracts to Capita until they meet their current agreements."
Capita is also expected to be handed the contract to run the proposed western extension of the congestion charge.
In October 2003 the firm was fined £1m for poor customer care, months after being given an extra £31m to improve the service.
A TfL spokesman said: "The main customer facing targets are all being met, many of them with good or very good scores."
Motorists are charged £5-a-day to drive into central London between 0730 and 1830 GMT, Monday to Friday.