Frustrated customers complained of problems accessing their account online and making payments. The issue lasted around 50 minutes and affected RBS, NatWest and Ulster Bank customers.
It was initially thought to be the result of an internal glitch, but the bank now claims it was caused by an attack on its servers. A spokesman insisted none of its customers' data was at risk.
"The issues that some customers experienced accessing online banking this morning was due to a surge in internet traffic deliberately directed at the website.
"This deliberate surge of traffic is commonly known as a distributed denial of service (DDoS) attack. At no time was there any risk to customers. Customers experienced issues for around 50 minutes and this has now been resolved. We apologise for the inconvenience caused."
It is not the first time RBS has been targeted. In December 2013, customers were locked out of their account for 12 hours as a result of a DDoS attack.