Passengers on London's transport network travelled for free after a technical glitch left people unable to use their Oyster cards.
A card reader fault affected buses and Tube stations earlier. Passengers were waved through barriers free of charge by Transport for London (TfL) staff.
The failure, linked to a fare increase, did not prevent people using the Tube, bus and rail networks.
TfL said the problem, which began at 04:30 GMT, had since been resolved.
'£250k in lost fares'
TfL said it was the first such problem with the card readers in 10 years and the glitch was linked to a 1.1% increase in fares overnight.
About 100,000 free journeys are believed to have been made between 04:00 and 10:00, which TfL estimated to be worth about £250,000 in lost revenue.
A spokesman said: "Customers should now touch in and out as normal. Anyone charged a maximum fare due to this issue will get an automatic refund credited to their Oyster card early next week. We apologise for this disruption."
People using pay as you go Oyster cards were able to travel for free while the card readers were down as the machines were unable to deduct the correct fare from the pre-paid cards.
Season ticket Oyster card users were unaffected as travel is not paid per journey.