Customers of Royal Bank of Scotland, Natwest and Ulster Bank are unable to access account services via mobile banking due to a systems failure.
App users experienced intermittent problems accessing mobile apps during Sunday afternoon and evening, with the bank able to restore services. However, difficulty accessing services has continued on Monday morning, with customers continuing to report problems via Twitter.
Owners of iOS-based mobile devices are believed to have be most impacted by the problems, with RBS claiming that Android, Windows and BlackBerry smartphones and tablets are currently performing better.
Other banking services - such as online banking, ATMs and payments - have not been affected.
An RBS spokesperson said: "We are aware that some customers are experiencing technical issues with our mobile banking. We’re investigating this and we are working to get it resolved as quickly as possible.
"In the meantime our customers can visit one of our branches, use online banking or call us via telephone banking as normal. We apologise to customers for the inconvenience."
The bank is currently investigating the issue, though the root cause has not yet been identified.
RBS has experienced numerous technology outages that have affected customer facing systems. In 2012 a failed upgrade to CA7 batch processing systems led to weeks of problems, with customers unable to access accounts or make payments. Following an investigation the incident eventually led to a combined 56 million fine from UK financial regulators.
The bank saw a recurrence of problems on the busiest shopping day of the year in 2013, and has seen a number of less disruptive outages since.
Mobile banking has become increasingly popular as in-branch transactions continue to decline. According to to the British Bankers’ Association, the number of mobile banking transactions has increased significantly in the space of 12 months, with smartphones and tablets used for 18.6 million transactions a week.