Nationwide has apologised after customers were unable to access their online and mobile banking services.
The building society said the outage was caused by a "technical issue", which occurred on Monday morning, but did not specify the numbers of people affected.
"We are currently experiencing a technical issue with our internet bank and mobile banking app and we apologise for any inconvenience this may cause.
"Customer service is the highest priority for Nationwide and the issue was identified quickly and we are working on restoring full service as soon as possible," the lender said.
Several Nationwide customers kicked up a fuss on social media, complaining of not being able to log in to their accounts and criticising the firm over its poor customer service.
One Twitter user wrote: "problems with internet banking? accounts missing..failed logins when details are correct etc #nationwide"
Another described the situation as "absolutely shocking", adding: "It's happening every week, a new problem. Their internet service is absolutely shocking! I have no accounts when I log in."
British banks are regularly hit with IT outages, and earlier this year customers with HSBC, NatWest, RBS, Lloyds and Halifax also struggled with online glitches.
Some NatWest customers said cash appeared to have "disappeared into thin air" as attempts were made to transfer money in April.