Thousands of HSBC customers in the UK have reported that they are unable to access mobile banking.
Downdetector, which tracks websites, showed more than 4,100 people had reported the banking app on their device was not functioning.
The problem comes on Black Friday, a day when many people will be making purchases due to the discounts offered by retailers.
The firm said it was "investigating this as a matter of urgency".
Some customers of First Direct, a division of HSBC UK, have also said they are affected - though others have suggested they are still able to use the app.
The firm has around 14.8 million customers across the UK, though it is not known how many of these use mobile banking.
The firm previously announced it would close 114 branches in the UK in 2023. At the time, it said this was due to an increase in customers using online banking.
There are hundreds of social media posts from frustrated customers affected by the outage, but one common theme is a frustration that throughout the morning HSBC's service status page has claimed mobile banking is "operating normally".
Some customers have signalled their frustration at being unable to move money between accounts so they can make online purchases, while others have said the app being down means they cannot verify purchases either.
Reports of the outage were first made at 07:00 GMT on Friday, according to DownDetector, with thousands of reports coming in by 08:00.
The exact number of people affected is unclear, because users must have a reason to check their online banking app to know if it is working.