Santander UK's mobile app was hit by a three-hour outage on Monday morning, leaving thousands of customers unable to access their accounts.
The app went down at around 7:00am, prompting more than 2000 reported issues on the DownDetector website, of which 96% related to mobile login.
Online banking services, card payments and ATM withdrawals were not affected by the "technical issues".
The bank's social media team spent the morning fielding complaints from frustrated customers:
The outage, which comes shortly after similar issues for Lloyds and NatWest customers, arrived on the day new research shows that Brits are less bothered about bank branch closures than people in the US and Europe.
The report, commissioned by consumer credit information provider CRIF, found that less than a quarter of UK consumers would choose their banking provider on the basis of them having a branch nearby.