Barclays said it has fixed problems which saw customers struggling to log in to their accounts and get through on the phone for much of the day on Thursday.
The bank said the problems, which started at around 10am, had affected its website and telephone banking.
Customers had been able to use the mobile phone app, although some mobile banking features had also been affected.
Barclays, which has around 24 million customers, confirmed late on Thursday afternoon that the problems had been fixed.
A Barclays spokeswoman said: “We’re very sorry about the technical problems our customers have experienced today.
“Everything is now back up and running, and we’re really grateful for customers bearing with us.”
Earlier, people had vented their frustration on Twitter, with one user saying: “It is driving me mad at the moment.”
Another said: “Their phone lines are down and online banking not working… thanks guys!”
The latest banking problems follow TSB’s huge IT meltdown earlier this year, following a botched IT switch which saw customers locked out of their accounts.