Barclays customers have reported problems using their cards in shops and withdrawing money from some cash machines.
The bank said it was "working to fix" a problem and advised customers to use other banks' cash machines.
It added that telephone banking and in-branch payments were also affected and apologised "for any inconvenience".
It is not yet known how many of Barclays' 15 million card customers have been affected by the problems.
In a tweet, the bank said: "We're still experiencing issues affecting Barclays Debit Card and ATM transactions. Our teams are working to get this restored."
: "Technical issues are affecting some digital services. We're investigating this and apologise for any inconvenience."
Technical issues are affecting some digital services. We're investigating this and apologise for any inconvenience.— Barclays UK Help (@BarclaysUKHelp) February 25, 2017
The problems come months after thousands of Barclays customers in the north of the UK had payments wrongly taken from their accounts.
Last October, Barclays said customers were refunded after they had duplicate debit card payments taken.
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,"
one customer, James.
Wondered why my card was declined when paying for lunch. Barclays servers have crashed. Brilliant.— james (@jamesyboi87) February 25, 2017
Are you a Barclays customer who has been unable to withdraw money or use your card in shops?Send your comments to firstname.lastname@example.org
Or you can contact us in the following ways:
WhatsApp: +44 (0)7525 900971
SMS/MMS: 61124 or if you are outside of the UK +44 7624 800 100