Barclays has apologised after customers reported problems using cash machines, debit cards and phone banking.
Some customers said they were unable to withdraw money from cash machines or use cards in shops. Online and telephone banking, and in-branch payments were also affected.
Customers were still reporting problems on Saturday evening.
However, Barclays said there had been issues with "some digital services" but that the cause had been fixed.
The BBC understands the fix is being staggered across bank's systems resulting in a delay before some customers get all their services reinstated.
It is not yet known how many of Barclays' 15 million card customers have been hit by the problems.
The bank said not all customers were affected but that issues had been reported across the UK.
The bank had earlier advised customers to use other banks' cash machines.
Several Barclays customers contacted the BBC to say they were experiencing problems.
In a tweet at 16:09, the bank said: "We're still experiencing issues affecting Barclays Debit Card and ATM transactions. Our teams are working to get this restored."
Barclays added: "Technical issues are affecting some digital services. We're investigating this and apologise for any inconvenience."
In a later statement sent to the BBC, a spokesperson said: "We're aware some customers may have had issues with some digital services today. We can confirm everything is now back up and running and customers can get money out of ATMs and card payments are going through.
"We apologise to any customers that were impacted."
The problems come months after thousands of Barclays customers in the north of the UK had payments wrongly taken from their accounts.
Barclays said customers had been refunded after having duplicate debit card payments taken from their accounts.
Many Barclays customers have tweeted about problems using their cards when out shopping or trying to access online banking on Saturday afternoon.
"Wondered why my card was declined when paying for lunch. Barclays servers have crashed. Brilliant," said one customer, James.
Nine-months pregnant, Ewa Wiczkowska was shopping in Aldi when her card was declined at the till.
The 26-year-old said she was spending the day with her daughter, before she is induced on Tuesday.
"I logged into my account to check and I do have enough money. I tried one more time, but it was declined again. I left Aldi so embarrassed.
"Instead of spending time in the park, going for lunch, buying a costume for Thursday's World Book Day at school, my daughter [and I] went back home.
"Our day was ruined, because I have no cash on me. That's the story of one very sad and disappointed Barclays Bank customer."
Another customer, Chris Sparks, who lives in Camberley, Surrey, said he was worried he would not be able to pay his council tax bill after his card was declined in a local Sainsbury's.
He said he was unable to buy food, fuel and a costume for his daughter for World Book Day.
"I can't do any of those things now. I also want to pay my council tax bill online and I'm worried that I will be fined if I can't pay it. Barclays cannot tell me whether this will last hours or days. I don't think Barclays do enough to keep their customers informed."