The Transport for London Oyster card system suffered a second technical problem in under two weeks this morning (25 July 2008).
Gates to all London Underground systems were left open after Oyster pay-as-you-go customers were again charged the maximum fare for their journeys.
The system enables passengers to use the swipe cards on most of the capital's public transport.
London Underground said the problem was due to "incorrect data tables being sent out by our contractor, Transys". The company said it was the same problem as last time, when around 60,000 cards were corrupted by a software failure.
A London Underground spokesman said no cards were corrupted by this morning's failure and anyone charged the maximum fare would get an automatic refund.
The company said in a statement, "The technical problem which affected Oyster readers at London Underground stations has now been resolved and all stations are fully functional. The problem affected pay-as-you-go users only.
"Steps are being taken to ensure that this does not happen again and we will undertake a full root cause analysis. TranSys regrets any inconvenience caused to Transport for London Ís customers.
"As Transport for London has made clear, Oyster cards are not being disabled and automatic refunds will be made to any customers charged maximum fares as a result of not being able to touch in and out at the beginning and end of their journeys. Oyster card holders need take no further action."