Nationwide customers across the UK have been hit by problems with their cards.
Transactions in stores and online were declined and some people were unable to withdraw cash from ATMs.
A Nationwide spokesman told The Independent the issue was a result of IT issues rather than "nefarious" activity like a cyber-attack.
The building society is working to rectify the issue and said it is hopeful that customers will now be able to withdraw cash without problems.
In a statement on its website, Nationwide said: "We’re really sorry, there’s a problem that’s causing some card payments to be declined online, at cash machines and in-store.
"We’re working hard to put this right as quickly as we can.
"Everything else is working normally.
"Thank you for your patience - we’re sorry for any problems this may cause you."
Angry customers took to social media to report problems.
Posting on Twitter, one user called Harriet wrote: "Nationwide is being terrible. Can't make any transactions or take moeny out because the system's down."
Other users said they were left "embarrassed" after their card was declined at store checkouts.
Gemma wrote: "Left starving on a nine hour shift cause I can't buy any food."
A spokesman for Nationwide told The Independent: "Due to a technical issue some customers are experiencing intermittent issues when attempting to pay for goods and services; we ask those affected customers to reattempt the transaction.
"All other services remain available, including the ability to withdraw cash from ATMs. Customer service remains our top priority and we are working hard to restore normal service. We apologise for any inconvenience caused."
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