A number of NatWest customers were unable to log in to the bank's mobile applications this morning, in yet another IT glitch at parent bank RBS.
RBS has apologised for the issue, and says it has fixed the problem for most users.
"We have been working hard to fix the problems affecting our mobile banking applications this morning," the bank said in a statement.
"Most of the applications are now back up and running normally although there are still problems affecting customers with the latest version of the iPhone and iPad applications.
"We're doing everything we can to resolve the problems as quickly as possible and apologise again to customers for any inconvenience caused".
The bank has not given any indication of the cause of the problem.
It is the latest in a string of IT problems at RBS.
Earlier this month, customers were unable to use cash machines or access their online accounts following a hardware outage.
Last summer, the bank suffered the mother of all IT outages that locked customers out of their accounts for weeks. That outage, triggered by a faulty upgrade to a batch processing system, cost the bank at least £175 million, mainly in customer compensation.
Earlier this week, RBS and NatWest added a feature to their business banking app that allow customers to chat to an advisor through instant messaging.
The significance of mobile as a platform for banking was evident in Barclays recent promotion of Shaygan Kherapdir, a former executive at US mobile telco Verizon, to its first ever group CTO.