Nationwide has apologised to customers for not being able to access their online banking services yesterday due to "planned maintenance" taking longer than expected.
It is the second time this month that customers have been locked out of their online accounts. However, the bank insists the incidents are not related.
"Unfortunately last night's planned maintenance overran, which meant customers could not access our online bank and mobile banking app," Nationwide said in a statement.
"Customers could still use ATMs and make Visa card payments."
It added: "The outage was an isolated incident linked to the planned maintenance."
According to the bank, the services came back online at around 7.30pm last night, after being unavailable for most of Sunday.
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.
Earlier this year, the UK's largest building society launched a new banking website aimed at improving functionality for customers using mobile devices, using a Sitecore content management system.
Sign up for CIO Asia eNewsletters.