London's Oyster travel smartcard system has failed for the second time in a fortnight, with many commuters enjoying a free trip to work during rush hour on Friday morning.
The system has failed at all London Underground stations and ticket gates have been opened to limit disruption while the problem was resolved.
Passengers who are charged the maximum £4 fare as a result of not being able to use their card at the start or end of their journey will be automatically refunded. Oyster cards have not been disabled as a result of the problem.
A Transport for London (TfL) spokesperson said the problem is believed to be the result of incorrect data tables being sent out by the Oyster card contractor, Transys.
The problem was resolved by mid-morning with card readers progressively coming back online.
The TfL spokesperson said: "We apologise for any inconvenience caused to our passengers."
TfL suffered another Oyster outage on 14 July. Unlike today's problem, some cards were disabled with around 65,000 travellers having to replace their cards.