HSBC has launched an investigation after systems failed at the weekend leaving customers unable to withdraw cash from ATMs. Online banking was also down.
"HSBC would like to apologise to those customers that were affected by issues relating to its ATM network over the weekend. A full investigation is currently underway to establish the main cause of the problem," said HSBC.
One reader said when he tried to withdraw money from a Lloyds ATM he was greeted with the message "Unable to perform this function, please contact your card holder." When he contacted NatWest he was told he had exceeded his daily card limit.
Fearing he had been defrauded he tried unsuccessfully to go online. He telephoned HSBC and was told all HSBC UK customers were unable to access their funds electronically and that it was working to fix the problem.
HSBC said the investigation could take several weeks.
Identifying the cause of the outage will be difficult because ATM networks are very complex, said Gareth Lodge, analyst at Towergroup.
"When you put your card in, it has to check it is valid and then go into a network to check it against other data such as bank balances. And it does this in near real time."
The investigation will take time because there are many possible points of failure because bank services share a plethora of different systems such as databases.
HSBC is going through major upgrades as it integrates its entire business onto a common platform known as One HSBC.
In February last year, an upgrade to a telephone banking system for business customers caused some HSBC customers problems.
HSBC said at the time that some customers could not complete some debit transactions. "The issue was caused when new coding was put into the telephone banking service for business customers."
Retail customers were affected because their details reside on the same database as business customers. "All services use the same customer database, so if you have a failure at one point it can bring down other points of the network," said a spokesman.