Nationwide customers have been unable to make payments online or in shops today.
The building society confirmed that a small number of debit cards were incorrectly declined this afternoon.
It issued a statement on social media as the bank said it was working to fix this as quickly as possible.
Replying to one customer on Twitter, it said: "We're sorry, a small number of debit cards are being incorrectly declined online and in shops at the moment.
"We're working to fix this as quickly as we can."
Outage tracker DownDetector shows the technical glitch started around 1pm today. According to its breakdown, most of the complaints (43%) are to do with online banking.
Some 32% of people are having trouble with transferring funds, while 25% are complaining about mobile banking.
In a tweet to Nationwide earlier today, one customer said: "Is there problems with account balances? I made a purchase but the money in still in my account and hasn’t been taken."
Another said: "I am trying to make a withdrawal from my Sky betting account to my Nationwide account its not going through.
"It is being declined from your side has there been some issues blocking this just I used my card to make a deposit."
A third tweeted: "Any issues with online banking? My account balance is higher than it should be and I can’t seem to make payments online. No messages on the app."
A fourth complained: "I need to go grocery shopping but can’t as my card declining every month there is something new."
Nationwide last had an outage in February 2022 building society were unable to pay their bills due to a software outage.
Nationwide spokesperson said there was a delay regarding faster payments as a result of a technical issue.
Transactions were temporarily queued while services returned to normal, the bank added.
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