Santander said its services have returned to normal after a technical glitch lasting much of Saturday saw customers unable to make payments.
The bank apologised for the inconvenience the problem caused.
Customers had reported being unable to 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 one person tweeted that they had an "unpleasant and embarrassing" experience when their card was declined in the queue as they tried to pay for petrol.
In a statement on Saturday night, a spokesperson said: "All of our banking services are now working as normal.
"We are very sorry for the inconvenience you've experienced today. If you need help, some of our branches will be open on Sunday from 10am to 12pm. We'll publish a list of these tomorrow."
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The bank had earlier assured customers that "no one will be out of pocket" as a result of the glitch.
Earlier in the day, the bank advised customers they could access cash from other banks' ATMs.
Gareth Shaw, head of money at Which?, said the technical issues will have caused "stress for many Santander customers".
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He added: "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."
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