Customers of Nationwide Building Society were unable to access mobile and online banking services for about two hours on Wednesday afternoon.
Other services such as ATMs and card payments were unaffected and the system. The problems have now been fixed, and both online and mobile services have returned to normal.
A spokesperson said systems were affected for 'a couple of hours', but was unable to give any details of the cause of the outage.
A number of Nationwide customers took to Twitter to highlight the problems. One, Andrew Smith (@AndrewS40941788) tweeted yesterday: "Why is Nationwide internet banking service not working today? Technicalproblemsthey say ~ here we go again!"
Responding to another customer complaint, Nationwide said from it official Twitter account: "Our Technology department are aware of this problem and are working hard to resolve this."
Nationwide currently has 2.6 million active users of its digital services, with almost half of its 620,000 logins each day made through its mobile banking app.
In its recent full year results, Nationwide highlighted plans to make further investments in digital banking systems, following the completion of a major project to re-platform its core banking system.
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