NatWest customers are understood to be unable to access their accounts via the bank's website and app due to an ongoing outage issue.
The outage in question appears to be impacting Microsoft services, with Heathrow airport and Asda among the businesses hit by the IT error.
According to the tech giant, Microsoft 365, which includes Outlook and Teams, might see delays.
The company's Azure cloud computing platform, which underpins large parts of the internet, reported a "degradation of some services, external" at around 4pm.
Banks, including NatWest, was recently impacted by bank app outages | GETTY According to Downdetector, there was a surge in complaints from customers over NatWest's banking app and website being inaccessible.
Taking to social media, customers complained to the bank about their services being "down" and are demanding answers.
One customer messaged NatWest on X: "Website and app is down. Any more affected? Of course, no customer service available."
Another shared: "Problems? off line can not get the page for the last 15 minutes."
Bank apps have been hit by outages in recent months | PALATEST DEVELOPMENTS
In response, NatWest stated: "The team are aware of this technical issues and are currently working to get this sorted, Please pop over to DM if you need any help right now."
Earlier this evening, Microsoft shared an update to affected customers and businesses who are dealing with the IT issue.
A spokesperson shared: "We are taking two concurrent actions where we are blocking all changes to the AFD services and at the same time rolling back to our last known good state.
"We have failed the portal away from AFD to mitigate the portal access issues. Customers should be able to access the Azure management portal directly.
A major bank is informing its millions of customers that they 'won't be able to use online banking' | GETTY Other major banks and building societies have also been forced to contend with similar service outage errors this year, including Lloyds Bank and Halifax.
Oliver Sissons, the SEO Director at Reboot Online, previously said: "In today's digital world, ensuring that your website or app offers a good user experience is more important than ever.
"Failing to do so risks consumers engaging less with your product and brand, and even seeking alternative options who provide a better user experience at the first chance they get and as soon as it makes sense to do so.
"In an industry as important as banking, one where you'd think the biggest brands would want to exude competence, confidence and security in every area, it is surprising to see so much negative sentiment surrounding the apps available.
There was a surge of bank app outage reports from 3:30pm today
|DOWNDETECTOR
"We do not have an ETA for when the rollback will be completed, but we will update this communication within 30 minutes or when we have an update."
"This perhaps explains in part why we have seen the rise of a good few digital banks in recent years where UX is a key focus from the outset."
GB News has contacted NatWest for comment.