NatWest and Royal Bank of Scotland customers were locked out of their accounts on Saturday morning as both banks' apps crashed.
More than 3,000 people reported issues on Downdetector, with many taking to social media to vent their frustrations.
One NatWest user complained: "I just can't log in. I have tried closing and re-opening the app".
Another pleaded: "How long will it be until it's resolved? I need to access some money."
NatWest acknowledged the problem on social media, stating their team was "working hard to get things resolved as soon as possible".
RBS also reported "connection issues" with their online and mobile banking services.
A NatWest spokesperson said: "Some customers experienced difficulty when attempting to log in and make payments via their mobile banking app and online banking this morning. This has been resolved and we apologise for any inconvenience caused".
The tech failure comes as a blow to millions of customers hoping to access cash before the weekend, especially as both banks have reduced their physical branch presence in recent years.
