Nationwide says it is working to fix an IT glitch that has hit online and mobile banking customers.
The building society said it was sorry for "any inconvenience" as customers took to social media to vent their frustrations.
The lender first acknowledged a "technical issue" early on Monday morning but said it was unable to give further details.
Sorry, Internet Banking & the Banking app are currently unavailable. We’re working to get them back up and running ASAP. You can get updates or register for a text message when things are back to normal here: https://t.co/Rz7WFLPXJP
— Nationwide UK (@AskNationwide) December 4, 2017
"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," its statement added.
There was no detail on whether all its customers were affected while some of those using Twitter to contact the company's support team suggested card payments were also being refused.
One user said: "It's happening every week, a new problem. Their internet service is absolutely shocking! I have no accounts when I log in."
Others pointed to anger about being pointed towards a Nationwide branch for help at a time when the sector is under fire for closing high street operations because of a customer shift towards online and mobile banking.
RBS, which has suffered some of the most high profile outages since the financial crisis, announced on Friday it was closing 259 more NatWest and RBS branches.
Lloyds and Yorkshire Building Society had also confirmed branch cuts earlier in the week.
British banks have been hit by regular IT outages – often on pay day.
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Customers of HSBC, NatWest, RBS, Lloyds and Halifax have been affected by glitches this year alone.