An investigation has been launched into how 60,000 Oyster smartcards were made inoperable over the weekend.
The travel smartcards were corrupted when swiped at ticket gates in the London Underground, causing major ongoing disruption to the capital's transport system.
The authority responsible for the cards, Transport for London (TfL), is working in conjunction with supplier Transys to find out how the cards could have been affected by the gates.
"We're currently investigating the cause of the incident with Transys," a TfL spokesperson told ZDNet.co.uk. "It's an ongoing investigation."
Cards started to be affected at 5:30am BST on Saturday, and the system was rectified by approximately 9:30am, according to the spokesperson.
TfL said in a statement that approximately 60,000 Oyster card users required replacement cards after the incident on Saturday morning while, as of Monday, 35,000 cards still needed to be replaced.
"Ticket offices are well stocked and we advise those passengers who have not yet replaced their cards to go to their nearest London Underground ticket office throughout today," stated TfL.
Third parties who supply Oyster card top-up services, known as 'Ticket Stops', were also affected. TfL warned customers that their cards may not have been topped up over the weekend if they had been swiped past ticket gates between 5:30am and 9:30am on Saturday. Ticket Stop retailers had also been affected, but were "coming back online", according to a TfL statement.
Earlier this month chipmaker NXP announced that it was going to sue Radboud University in Holland, to try to prevent the publication of a research paper detailing the cryptographic cracking of the Oyster smartcard. NXP produces the Mifare Classic chips used in the smartcard.