Royal Bank of Scotland will have to pay compensation to customers for the second time in less than a year after a hardware fault blocked their cards.
Frustration grew on Wednesday night after people saw their attempts to access funds blocked, or had their cards seized by ATM machines.
Online and phone banking services were also disrupted into yesterday.
The incident raised further concerns over the systems used by the bank's parent company following an IT problem during a major upgrade in June last year that affected RBS and Ulster Bank.
RBS Group apologised for the "unacceptable failure", which it said was due to an IT fault unrelated to last summer's computer meltdown that cost the group £175 million to put right.
The banking group said the hitch was cleared up a couple of hours later, but some customers were still reporting problems on NatWest's website yesterday.
It urged customers who have been left financially disrupted to get in touch, and it promised they will not be permanently out of pocket.
RBS Group said: "This problem was caused by a hardware fault and was not related to the issues we experienced last summer.
"It was much easier to fix, though clearly an unacceptable failure.
"Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."
A spokeswoman for the group said claims will be looked at on a case-by-case basis to work out if people should be compensated for any extra costs.
Customers will not be automatically entitled to a payout just because they had problems, and those who believe they are entitled to some cash should keep evidence such as receipts.
Some customers have reportedly been compensated already.
It is unclear exactly how many of the 17 million personal banking customers of NatWest, RBS and Ulster Bank had been affected, although many people vented their frustration online.
One Twitter user said: "Natwest, you left me with no dinner tonight, and left me walking home in the rain!"
One customer wrote: "Tell us what's happening!
"You put up all sorts of irrelevant guff on the online banking service, which we have to click to accept, but not what we really want to know – the status of your banking systems."
RBS has been investing in technology to make it easier for people to bank with the group, including introducing more than 200 ATMs and cash deposit machines.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article