The Ulster Bank has apologised to its customers amid reports from customers of being unable to access accounts or withdraw money.
It came months after a computer meltdown that left millions of customers unable to withdraw cash.
RBS and its subsidiaries NatWest and the Ulster Bank issued apologies via Twitter after customers reported problems with cash machines and cards being declined, accessing their accounts online and via telephone.
A statement tweeted by accounts run by all three banks said: "We are aware of the problems our customers are having and apologise, we will provide more information as soon as we have it."
Stephen Hester, the chief executive of RBS, which is 80% state-owned, was forced to apologise last June after millions of customers were left unable to view an up-to-date balance, payments such as direct debits for bills were not made and some wages were not received following a software update.
Customers took to Twitter to vent their frustration. One, Mark Hillman, from Maidenhead, Berkshire, tweeted from his @Hillmania account: "Natwest whole system is down! No ATM's, no online banking and cards WILL be declined. Just found that out the hard way. Pls RT."
Ulster Bank said via Twitter:
"We are aware of the problems our customers are having and apologise, we will provide more information as soon as we have it."— @UlsterBank_Help March 6, 2013
RBS was not immediately available for comment on what was causing the problems.
NatWest customer Stuart Keel, from Cornwall, told Sky News he tried to get cash out of an ATM but it was not working. "We went to the supermarket thinking we could use our cards in there, no problem," he said. "While we were walking around I was using my NatWest app and it wasn't working at all."
He said his card was then declined at the checkout. "I thought 'there is something not right here'," he added.