"It is impacting HSBC UK customers only - there is no impact to First Direct or M&S Bank customers," it said, referring to other divisions of HSBC UK.
Marius Acsinte, 34, told the OceanNewsUK his HSBC account was unavailable, causing him several problems.
"I'm not able to open the app to pay my rent due today," he said.
"I was expecting the bank would send a message or an email explaining why, but instead I had to find out online.
"It doesn't matter if it's Black Friday tonight or not, I'm not able to use my money and nobody informed me about why that's happening."
Others have shared their frustration at being unable to make card payments online, because they cannot verify purchases using the app.
But the firm said customers affected in this way could "opt for a one time passcode via SMS".
The exact number of people affected by the problems at the bank is unclear, because users must have a reason to check their online banking app to know if it is working.
HSBC has around 14.8 million customers across the UK. Though it is unknown how many of these customers use mobile and online banking, the firm estimates that nine out of 10 transactions at the bank are made digitally.
The bank 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.
Frustrated customers
Users on X, formerly Twitter, responded to HSBC UK's post on the platform with complaints - some saying they were unable to verify payments for Black Friday purchases or settle bills on payday.
"I got paid today and cannot pay any bills! Sort it out HSBC," complained one user.
Others signalled their frustration at being unable to move money between accounts so they could make online purchases.
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.
However, the bank did not update its service status page, which provides information about the availability of digital services like online, mobile and phone banking, until around 11:00 - something which also annoyed customers.
"It's the lack of communication from the start of the issue that's most worrying. You haven't even updated your own service website. I'll definitely be moving some of my funds due to the poor communication," wrote one person.
Sam Richardson, deputy editor of Which? Money, said the outage would cause "a real headache" for customers.
"We strongly advise customers that have been left out of pocket to keep evidence of extra expenses they may have incurred as a result of the outage, so they can be claimed back from HSBC," he said.
"People want a bank they can depend on, and if IT outages become a regular occurrence, consumers could be tempted to vote with their feet and switch to an alternative provider - particularly with a lot of tempting switching incentives on offer at the moment."
