Santander has apologised to customers for a technical glitch and assured people they will not be left “out of pocket”.
The problem, which has lasted several hours, saw customers report that they could not use the app or their online banking.
Consumer organisation Which? said some people had reported not being able to buy food at the supermarket while the issue persisted.
Others on Twitter said they could not transfer money to their current account, while one person said they had an “unpleasant and embarrassing” experience when their card was declined in the queue as they tried to pay for petrol.
The bank said in a statement on Saturday evening that “key services” were “coming back on stream”.
A spokesperson added: “We continue to work on full recovery by the end of the day. We sincerely apologise for any inconvenience caused.
“No one will be out of pocket and from Monday we will make available via our website and social media channels details of how impacted customers can make a claim.”
Earlier in the day, the bank advised customers they could access cash from other banks’ ATMs.
Gareth Shaw, head of money at Which?, said: “These technical issues will be causing stress for many Santander customers – with people reporting that they have been unable to make online payments or in some cases purchase food in their local supermarket.
“Customers can incur fines, penalties and fees when they’re not able to access their finances, so the bank must offer compensation to all those who have been impacted in this way.
“These problems demonstrate why it is vital that banks invest to ensure their systems are up to the task of protecting their customers’ accounts and maintaining the services they rely on.”