Transport for London has apologised for the Oyster card fault
Thousands of people using London's public transport network may find their electronic Oyster card no longer works after a fault hit the system.
The system was inoperable for at least five hours on Saturday. Some cards used during that time have since stopped working or incurred a fine.
The cards are used as payment on buses, Tube, tram and Docklands Light Railway.
Transport for London apologised and said people with a faulty card could get a replacement from Tube stations.
A spokesman said: "Customers whose cards are not working are advised to go to their nearest London Underground ticket office where they will be able to exchange their card for a replacement.
"A small number of Freedom Pass and Young Persons Oyster card holders may also have been affected.
"They will be required to apply for replacement cards through their relevant local authority and through the young persons travel card helpline respectively.
"Our staff will be instructed to allow these card holders to continue to travel whilst replacements are issued."
It follows a computer fault which lasted from about 0530 BST to 1030 BST on Saturday, rendering cards inoperable, although several bus routes across the city were still unable to read cards much later in the day.
Ticket barriers were left open while the problem persisted to allow passengers pass through without delay.
Free bus travel continued on the roads for longer because individual buses had to return to their garages for their systems to be reset.
Machines used to place funds on the cards were also affected by the fault.
Transport for London (TfL) apologised and said Oyster faults were "very rare", adding that a problem of this nature had not happened since March 2006.
Passengers touch the electronic card on the reader on entry or exit to a station, or when entering a bus.
Any passenger who entered the underground or DLR system while the system was down and finished their journey after it was repaired may be charged the maximum fare.
In a statement issued after the fault occurred, TfL said: "All passengers who incur a maximum fare on Saturday 12 July will be given an automatic refund from Tuesday. They do not need to take any action.
"We are investigating the cause of the problem, will ensure that any necessary refunds are honoured and apologise to our passengers for any inconvenience caused."
In May, London's transport commissioner Peter Hendy said 17m cards had been issued since the introduction of the Oyster system in 2003.
He added that it had successfully sped up journeys and reduced queues on the city's public transport network.
Passengers can currently use Oyster cards as a travel card to cover fares for periods up to one year, as well as to cover single journeys for travel to London destinations including overground stations.
People using the cards can also use them on a pay-as-you-go basis by topping up the amount of money on it to cover journeys of varying distances.
Commuters within London travelling on First Great Western (FGW) trains will be able to use pay-as-you-go Oyster cards from September.