Customers of NatWest, RBS and Ulster Bank are currently unable to access their bank accounts through the group’s online and app platforms.
Since around 5am on Friday, account holders have been reporting problems with the services.
The app says it is “unable to establish a connection”, while the bank’s website displays a message saying that online banking is “temporarily unavailable”.
A spokesman said “We are aware of the problem and are working to fix it”.
It added that “Customers can still use telephone and ATMs.”
Customers have been sharing their frustration on social media, with a number pointing out that the problems have arrived at a terrible time – payday.
Fed-up customer Paul Murphy told the BBC: “This is just what you need as the weekend approaches and bills to pay.”
The banks had a similar problem in April last year, when their banking apps stopped working.
RBS Group, which owns NatWest, reported last year that it had more than four million customers who use its mobile app services.
This year has proven to be a terrible one for banking customers with a number being locked out of accounts after their bank has been hit by technical issues.
Barclays customers were locked out of their accounts online for several hours on Thursday.
Meanwhile customers of online challenger bank Cashplus – which targets people with poor credit histories – were unable to access their accounts, make cash withdrawals, or make or receive payments earlier this week.
Earlier this year TSB’s huge IT meltdown led to weeks of pain for customers and the eventual resignation of chief executive Paul Pester.