NatWest's online banking has gone down, according to tech troubleshooting site Down Detector.
A number of customers have taken to social media to report issues with the site and app, with many claiming they are unable to check their balances.
One customer tweeted NatWest's official help account, writing: "Is Bankline down?! Been trying to get on my business account all morning! Can't access via desktop or mobile!"
Another wrote: "What's up with Bankline? Of all the days in the year for it to be playing up..."
On its Twitter, NatWest said: "Some customers aren't currently able to view their credit card information on our mobile app or Online Banking.
"We're working hard to fix this, sorry if it's causing you any issues."
It also directed customers towards its guidance and website service status page.
This is the latest IT problem to hit NatWest and its parent Royal Bank of Scotland.
In September, both bank's websites went down on the same day, leaving users unable to manage accounts online.
Despite this, the lenders said their mobile apps were still working.
But they are by far the only banks to have IT issues in recent months.
In October this year, the Lloyds and Halifax online banking services went down.
Yesterday, the HSBC website and app went down, as furious customers slammed the firm.