The Oyster card network for London public transport crashed in the early hours of this morning after a failure in the IT monitoring system.
Transport for London said that problems with its 24-hour IT monitoring system resulted in the disabling of its Oyster card electronic scanning system. About 2.2 million people use the Oyster card system each day.
A spokeswoman for Transport for London said a problem occurred with the IT systems monitoring application at 4.30am on 10 March.
"There was a four-hour turnaround - by 8.30am we had identified and started rectifying the cause of the problem.
"We are currently working to rectify the problem," said the spokeswoman.
"There are no delays to passengers; they just show their Oyster card at the gate, and there are signs up to indicate this."
At 10am, TfL said 90% of the Oyster network was back up.
"Now we are working out how to avoid this happening again in the future," said the spokeswoman.
The Oyster smartcard stores a passenger’s season ticket or pay-as-you-go information, and is recognised by card readers across London, at London Underground and Docklands Light Railway stations, on buses and at tram stops.
Last week Transport for London announced that Oyster card users may soon be able to buy newspapers, milk and car parking with their smartcards.