More than a third of all congestion charge revenue comes from fines, new figures have shown.
'Give motorists an extra day to pay'
In the financial year up to October 2004, £64.2m was made from charges, while income from fines was £38.5m.
Liberal Democrats have responded by calling for drivers to be given an extra 26 hours to pay the congestion charge before being fined.
Transport for London said a number of options to reduce revenue from penalty charges were being considered.
Drivers currently have until 10pm to pay the £10 charge otherwise a penalty notice charge of £100 is served.
Liberal Democrats claimed the figures showed Mayor of London, Ken Livingstone, relied on people forgetting to pay the charge to balance his books.
Lynne Featherstone, London Assembly Liberal Democrat transport spokeswoman, said: "The current charge fining structure appears to have nothing to do with reducing congestion and everything to do with raising a bob-or-two.
"It is time the Mayor stopped clobbering motorists who intend to pay the charge but miss the 10pm deadline and give them an extra day to pay."
A Transport for London spokesman said improvements to payment systems and better customer understanding were some of the changes being brought in which were expected to result in fewer penalty charge notices.
Figures from the previous financial year show income from fines was £55.6m - 5% less than this year.
Transport for London said a rise in the penalty charge from £80 to £100 in June accounted for this increase.