A quarter of drivers in central London who have been fined for not paying the congestion charge will challenge the penalty, according to transport chiefs.
Traffic levels have fallen since the charge began
Transport for London (TfL), which operates the £5 a day scheme, said it expected between 20% and 25% of motorists to appeal against their £80 fines.
TfL said many would cite errors such as putting in the wrong data and buying a ticket for the wrong day.
Congestion charging began on 17 February and has resulted in a fall in traffic levels of about 20%.
TfL said 100,000 people had failed to pay the charge in its first weeks and appeals have to begin within 28 days.
TfL will look at any evidence, such as any photographs of the car inside the congestion zone.
If there is still a dispute after TfL has re-examined the claim, then an independent adjudicator could be called in to make a final decision.