NatWest, RBS and Ulster Bank have all suffered from major website outages that have prompted a deluge of complaints from customers locked out from their online banking services.
The problems were first reported at around 9am on Tuesday morning, with systems still offline at the time of writing (15:00 BST).
"Our website is still unavailable and we're working hard to fix it," a NatWest Twitter post reads, which has been reposted three times over the course of five hours.
The same message has also appeared numerous times on RBS' twitter feed.
The three banks, all of which are owned by the Royal Bank of Scotland Group, have advised customers they are able to turn to mobile apps to access their accounts, however, some customers have complained of poor performance and unresponsive services.
One NatWest customer said: "It's like the money is disappearing".
Thousands of bug reports have been submitted by users since the early hours, according to outage monitoring site Downdetector.
An RBS spokesperson told the BBC: "We are aware that customers are currently experiencing issues accessing our websites and online banking login pages. We apologise for the inconvenience caused."
NatWest has suggested customers try accessing their online services using nwolb.com, but customers have also reported problems accessing this page too.
NatWest alone is said to serve more than 7.5 million current accounts, as well as more than 850,000 small business accounts. When combined with RBS, the total number of potentially affected accounts could be well over 25 million.
It's still unclear what has caused the issue, but the problem appears to be the same across all three banks. There is currently no timeframe on a potential fix.
NatWest and RBS suffered a similar IT glitch in 2014 when mobile banking services were knocked offline for almost a full day.
