A breakdown in London's Oyster card network left tens of thousands of bus and train passengers needing replacement cards today.

With long queues forming at ticket offices, some passengers were allowed to travel free to minimise disruption.

About 65,000 people had their cards corrupted when they swiped during problems with the system on Saturday. People who topped up their credit also found their cards no longer worked.

More than 30,000 of the affected Oyster cards had been replaced by lunchtime today. Those yet to swap their card are being asked to do so as soon as possible.

A Transport for London (TfL) spokesman said: "The vast majority of passengers have travelled without any disruption this morning and London Underground (LU) staff have minimised the delay to passengers with cards that are not working.

"Less than 1% of the six million regular Oyster card users required replacement cards after the incident on Saturday morning.

"We are replacing affected cards and there are now less than 35,000 cards that need to be replaced.

"If this has not been practical during this morning, LU staff, and London bus drivers, have allowed these passengers to travel.

"Ticket offices are well stocked and we advise those passengers who have not yet replaced their cards to go to their nearest LU ticket office throughout today."

Some people were wrongly charged the maximum journey fare because the system failure meant they could not swipe out when they finished travelling on Saturday.

The spokesman said anyone who paid the maximum fare on Saturday will have the sum credited back on to their card next time they swipe at their regular station.

"They don't have to do anything," he said. "That will automatically be credited back to their card."

He admitted some people will get a refund even though they rightly paid the maximum fare but added that the "vast majority of people will have been caught up in this problem".

He added that the TfL and Oyster provider, TranSys, has launched an investigation to find out what caused the problem.