The banks helpline on Twitter received dozens of messages from customers unable to log-in for online banking and the banks mobile app.
RBS, NatWest and Ulster Bank customers were affected by the problem, which lasted around 50 minutes.
It is not the first time the banking group has had IT problems. In 2014 it was fined GBP 56 million by regulators after up to 6.5 million customers from NatWest, RBS and Ulster Bank were prevented from accessing their accounts in 2012.
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