A technical problem with Oyster card readers at London Underground stations meant that gates had to be left open this morning to allow passengers to travel without 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 as 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 be used as normal."

TFL said that it will automatically refund any passengers who had been charged the maximum £4 fare as a result of not being able to touch in and out at the beginning and end of their journeys this morning.

"This problem, like the recent issue, resulted from incorrect data tables being sent out by our contractor, Transys," the spokesman said.

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