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Customers of RBS, NatWest and Ulster Bank are currently unable to access their accounts through the banking group's online and app platforms.
Since around 05:00 on Friday, account holders have been reporting problems with the services.
An RBS spokesman said "We are aware that customers are experiencing issues and are working to fix it".
He added that: "Customers can still use telephone and ATMs."
According to the group's latest annual report it has 19million customers in the UK and Republic of Ireland with 5.5million active mobile app users.
Many of them have been sharing their frustration on social media, with a number pointing out that the problems have arrived at a terrible time - payday.
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.
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.