The problems, which began at around 1230 GMT (8:30 a.m. EDT), hit telephone and online banking services and caused frustration at retail outlets as embarrassed shoppers were forced to abandon purchases at the till.
Barclays' telephone banking centers were inundated with calls from customers, many of whom were angry at the lack of information. A Barclays spokeswoman was unable to elaborate on the cause of the problems.
"We became aware of a problem earlier in the day," she said. "We apologize to customers for any inconvenience."
At 1915 GMT, access to online accounts was still being denied and calls to the helpline suggested the problems were ongoing.
Barclays is Europe's sixth-biggest bank by market value. It signed a two-year deal with NCR Corp on Aug 16 for the management and maintenance of its UK network of ATMs.