Thousands of users of London's cycle hire scheme have been overcharged, Transport for London (TfL) has admitted.
The initiative was launched in central London on 23 July.
TfL said a "couple of thousand" of the 40,000 cyclists registered for the scheme had been affected. Between £1 and £10 has been wrongly billed.
Those affected have been contacted and any incorrect charges are to be refunded, TfL said.
A TfL spokeswoman said: "Steps have been taken to ensure this error does not happen again and we apologise for any inconvenience this may have caused."
About 5,000 cycles are available across 330 locations in central London.
Cyclists have to pay an access fee from £1 a day plus usage time, with the first 30 minutes of cycling free.