A hardware problem brought down the Royal Bank of Scotland's online, telephone banking and cash machines at the weekend, the bank has confirmed.
The Royal Bank of Scotland Group (RBS), which includes NatWest bank, has more than 22 million personal customers in the UK.
A spokesperson said RBS was "extremely sorry for the inconvenience caused to our customers due to the technical problems we experienced with our systems over the weekend".
The banks systems crashed late on Friday night, with services restored over the course of Saturday. Service to cash machines was restored in the early hours of Saturday morning, the spokesperson said.
RBS refused to give details of the hardware problem. "It is not appropriate to go into further details, due to security considerations," the spokesperson said. But he added that the problem had been an internal breakdown and not the result of an attack.
"Customers were able to use bank cards in retail outlets throughout this time and, where available, could request cash back," he said. "We have an extremely good record of service availability and this was an incredibly rare occurrence."
Last month, RBS become the second high street bank to give out handheld chip and PIN readers to customers.