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"There is currently a technical problem with Oyster readers at London Underground stations which is affecting Oyster pay as you go cards only," explains the TfL website. To minimise confusion, gates at all underground stations have been opened.

A similar problem occurred two weeks ago when the network crashed, leaving 65,000 Oyster cards permanently corrupted. However, no cards will be similarly affected following today's crash, claims TfL spokesperson, Dan Maskell.

Last month researchers from Radboud University announced they had hacked the Oyster card system and used cloned cards to gain

Plans to publish details of the hack were briefly halted when the makers of the chip used in the system sued the group, although a judge ruled earlier this week that the researchers could go ahead.

While the matter was being fought in the courts, details of the hack briefly appeared on the website Wikileaks.

However, the recent problems are "completely unrelated" says Maskell, claiming instead that the fault lies with one of TfL's contractors.

"We believe that this problem, like the recent issue, resulted from incorrect data tables being sent out by our contractor, Transys," claims Maskell.

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