The group has apologised as customers at its Royal Bank of Scotland, Natwest and Ulster Banks were left locked out their accounts when they tried to access its online banking or mobile app.
The problem was initially thought to be a glitch affecting only Natwest customers, but later emerged as being widespread across the RBS banking group.
The bank said it is “working hard to fix” the problem.
In a statement sent to customers it said: “We are sorry if you are having trouble getting into our online banking or mobile app.
“We are working hard to resolve the issue. You can still access your account through telephone banking or alternatively you can visit one of our ATMs.”
The glitch comes a day after some Barclays customers were left struggling to log into their accounts due to a technical problem.