The mobile banking app and internet banking were down at Britain’s biggest building society.
Nationwide’s online services had 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 were working normally, and customers could 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.
At 2.44pm 1,900 users reported issues with Nationwide services on the Downdetector website, but the service was back working around 4pm.
The issue of bank outages has been examined by the influential Treasury Committee of MPs after services at several high street lenders were disrupted for two pay days in a row.
In response to a February letter from the committee, Nationwide said it had seven incidents of IT failure, impacting customers for 1,577 minutes in 2023, which rose to 11 incidents in 2024 for 2,649 minutes.
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While the overall number of customers impacted could not be recorded, Nationwide said 1.99 million transactions were affected across the two years, which represents approximately 0.03% of all transactions over the period.
The failures were caused by “technical faults in hardware or software”, or as a result of an outage following a planned upgrade or change to Nationwide systems, the March letter read.
