A power failure disrupted branch computer systems at Halifax and Bank of Scotland and left customers unable to access cash point machines and online banking services.
An external power failure at a datacentre used by HBOS in Yorkshire left customers unable to access banking services on Saturday morning.
HBOS, which apologised to customers, said it managed to restore all banking services by 14:30 on Saturday, while internet banking was fully restored by 18:30.
"Due to external power disruption in Yorkshire, which impacted one of our IT centres there, some Halifax and Bank of Scotland systems were affected on Saturday morning. Power was restored to the centre via our back-up generators in the late morning and all systems, including cash machines, were running normally as of 14.30," a spokesman said.
Bank branches opened as normal on Saturday morning, but it was only possible to withdraw money at the counter service, according to a report on the BBC. Customers were limited to a maximum withdrawal of £200 as staff were unable to check the balance of customers' accounts and had to write transactions down on paper.
HBOS cash machines were out of action, and customers said their cash point cards were rejected by cash machines belonging to other banks, and by some shops.
"We continued to experience some problems with internet banking, but these were resolved by 18.30 on Saturday. We apologise to customers for the disruption and inconvenience caused," HBOS said.