Some 2.2 million people use the Oyster card ticket system each day.
Faulty data in a daily download which is sent out across the system to update smartcard readers caused the units to fail on 10 March. The download carries information about cards that have been lost or stolen to enable readers to block them.
All readers at London underground stations and Docklands Light Railway were affected, a TfL spokeswoman said. "Now we need to look into why the data caused the readers to stop functioning," she added. The fault was corrected by midday.
The Oyster smartcard network, launched in July 2003, is run by the TranSys consortium, which comprises EDS, Cubic Corporation, ICL (Fujitsu) and WS Atkins.
Oyster cards store passengers' season ticket or pay-as-you-go information. TfL this month invited IT companies to collaborate on a project to allow Oyster card users to buy newspapers, milk and other low-value items with their smartcard.
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