Nationwide has apologised to its online and mobile banking app customers who could not use their accounts when maintenance work over-ran.
Customers complained on Twitter that they were not able to carry out transactions.
Nationwide said the problem lasted from midnight until around 7.30pm, when its online bank and mobile banking app were brought back online.
Nationwide said it "typically has around 500,000 customers who log on online to use these services on a standard day".
Customers were still able to use cash point machines and make Visa card payments.
A spokesman said: "Unfortunately our overnight planned maintenance has overrun and affected customers accessing our online bank and mobile banking app.
"We apologise for the inconvenience caused to our customers. The online bank and mobile banking app are now up and running."