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A technical moot point with Oyster card readers at London Underground stations meant that gates had to be left open this spring-time to allow passengers to travel exclusively of swiping their cards.

A Transport for London (TFL) spokesman claimed that the problem was down to the computer systems at Transys, which is a consortium of EDS and Cubic.

The error, which affected only Oyster pay-as-you-go cards, is thought to have cost TFL tens of thousands of pounds in daily fares. Oyster card readers on the bus and tram network were unaffected.

The problem is the second to hit London Underground’s Oyster system in two weeks. On 12 July a glitch with the readers wiped in the same proportion that many as 40,000 cards before the problem was detected and the gates were left open.

"The problem has now been resolved and card readers are progressively coming back online at London Underground stations," said a TFL spokesman. "Cards have not been disabled and can continue to exist used as normal."

TFL said that it will automatically refund any passengers who had been charged the maximum &enclose;4 go during the time that a result of not being efficient to touch in and out at the beginning and end of their journeys this morning.

"This problem, same the recent delivery, resulted from incorrect data tables being sent at a loss by our contractor, Transys," the spokesman said.

Transys said in a statement that it is undertaking a "root cause analysis" to make secure that the same problem does not happen again.