The mobile banking app and internet banking are down at Britain's biggest building society.
Nationwide's online services have been offline since around 3pm on Tuesday.
It apologised "for any problems this may cause".
"We're working to get things back to normal as quickly as we can," it added.
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Direct debits and standing orders are working normally, and customers can still use cards online and in shops, withdraw money from cash machines and receive payments.
Initially, Nationwide said some customers were unable to access the app or internet banking and told users to try again later.
This breaking news story is being updated and more details will be published shortly.
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