Motorists may be paying over the odds by using unofficial websites to pay the London congestion charge.
Transport for London (TfL) charges £10 per day for the congestion charges, but the Advertising Standards Agency (ASA) says that copycat websites are charging customers as much as £16.
TfL estimates that about 1,000 people a day are using the unofficial sites and are often paying over the odds without knowing.
RAC spokesman Simon Williams says: "With the cost of motoring already sky high there's only one thing worse than paying the congestion charge... paying it twice. Rogue websites are obviously keen to exploit public uncertainty about how to pay the congestion charge, so motorists should be clearly warned to only visit the official website."
Last week the ASA told one website, paylondoncongestion.co.uk, to make it clearer that it had no affiliation to TfL. A sponsored search advert was also investigated following claims it misled users.
In its ruling the ASA said: "The ads must not appear again in their current form. We told paylondoncongestion.co.uk to ensure that their ads made clear that they were not affiliated with TfL, and to specify the amount of their fees at or before the point at which consumers were invited to enter their personal details."
As well as charging £16 for the congestion fee, paylondoncongestion.co.uk customers have to fork out £20 to pay for the previous day instead of the TfL charge of £12.
Paylondoncongestions.co.uk says the charge is a "service fee for checking and processing your application".
Many unofficial websites justify their extra cost by claiming it provides extra services such as email confirmation and a dedicated helpline. However, TfL provides these services at no extra cost.
Garrett Emmerson, TfL's chief operating officer for surface transport, said: "TfL will be taking steps to seek to ensure that unofficial websites and search engines who allow them to advertise are not misleading the public.
"We will continue to pursue these avenues but in the meantime we urge motorists to take care and only use TfL's website."
Copyright Press Association 2014