A computer fault caused thousands of Oyster smart cards to malfunction on London's tube system over the weekend.
About 65,000 cards were corrupted and stopped working when they were swiped during technical problems on Saturday.
Tube workers handed out replacement cards and to prevent long queues some passengers travelled free of charge while London Underground drafted in extra staff to cope with the situation.
According to Transport for London (TfL), more than 30,000 affected cards have so far been replaced.
Retail machines used to top-up the cards were also affected, but are now back online.
An investigation has been launched into the problem and TfL is expected to release a statement later this week.
By staff writer
