The bank, a wholly-owned subsidiary of Royal Bank of Scotland, was inundated with complaints when its app failed to fully load on users’ mobile phones between 9.05am and 9.45am this morning.
Responding to the complaints, the bank’s customer service department tweeted: "Due to the overwhelming popularity of our app, we have experienced issues, which hopefully will be fixed soon."
A subsequent tweet stated: "Sorry if you had trouble getting into Mobile Banking. Our service is now running as normal, thanks for your patience."
An RBS spokesman added: "We’ve had huge demand for our app this morning, which led to some customers experiencing the app running slowly for half an hour. We apologies for the inconvenience caused."
However, Michael Allen, director of application performance management, Compuware, said the excuse was a feeble one.
He added: "’A higher number of users for the mobile app simply is not a good enough excuse for slow performance.
"Consumers expect fast and reliable access to banking services on their mobile devices and any deviation from this results in the kind of negative viral publicity Nat West is seeing today. Mobile apps are just as important as online banking from a consumer perspective and, therefore, the same safeguards ensuring performance need to be deployed.
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"Good app performance doesn’t just happen – it takes a conscious effort throughout every stage of the design and development process, and then needs to be monitored and managed proactively."
Last week, customers of RBS also experienced issues with the app, which has two million users.
