Nationwide Building Society's suffered online banking services outage on Tuesday, with customers unable to access account information for most of the day.
The building society's customers reported on Twitter that access to online and mobile banking was unavailble, though ATMs remained unaffected.
"Nationwide online banking not working, can't make payments," said one customer, with another highlighting problems with "both the app on my phone and online".
The service resumed later that afternoon after the issue was resolved Nationwide said from its Twitter account yesterday. "We can confirm the problems to our Online Bank have been rectified but we'll continue to closely monitor all services," it said via its @AskNationwide account.
In a statement the building society, which has which has around four million customers, also apologised to customers for any inconvenience as a result of the outage.
"During the day we have continued to offer a full ATM service and card payments and purchases have been unaffected. Nationwide Branches and Contact Centres have also continued to operate throughout the day," a spokesperson for the building society said.
"We are pleased to confirm that the problems have now been rectified but we will continue to closely monitor all services. We sincerely apologise to any customers who have been inconvenienced today. We very much regret falling short of our normal level of service."
However Nationwide was unable to provide any further information as to the cause of the technical problems.
A number of UK banks have been by IT failures affecting account services in recent months, with Lloyds customers hit earlier this month. A major IT failure at RBS and Natwest last year also led to the recent launch of an investigation by UK regulator, the FCA.
Nationwide overhauled its ageing online banking systems in 2011 to help improve usability, in a project led by IBM. It was revealed by Nationwide that it was able to launch its online service following testing conducted entirely in a virtual environment.
Computacenter subsequently helped deploy a new data centre platform as part of the transformation project, based on IBM servers and VMware vSphere virtualisation.