HSBC has apologised after thousands of UK customers reported they were unable to access mobile and online banking for more than 24 hours.
The bank was flooded with complaints on social media, with users saying they have been unable to pay bills or make discounted Black Friday purchases online since Friday morning.
One customer told the BBC the outage left him unable to pay his rent.
HSBC apologised for "the inconvenience".
"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.
The problems started shortly after 08:00 GMT on Friday, with the bank saying they were "investigating this as a matter of urgency".
Around 09:00 GMT on Saturday, HSBC posted on X saying the glitch had been resolved and online and mobile banking services were back up and running but that "some services maybe slower as customers log in".
'I had to find out online'
Marius Acsinte, 34, told the BBC 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."
Judi Sutherland, 76, said the problem particularly hits her as someone who lives in a rural area.
"I live in the Highlands of Scotland where branches have been systematically and carelessly closed throughout the region," she said.
"This loss of service means that some would have to make a journey to Aberdeen to access their accounts - a round trip of some 450 miles.
"The online service from HSBC has been excellent in the past but this outage does underline the vulnerability and dependency of rural communities on such services," she said.
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. 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.
'Sort it out'
Social media users on X 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.
Despite the problems starting early on Friday morning, 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 GMT - 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."