The latest computer glitch could be the "final straw" for Ulster Bank customers, the finance minister Simon Hamilton has warned.
Mr Hamilton was speaking after the IT problem – the third in 18 months – meant high street and online shoppers were unable to use their debit or credit cards to make payments for several hours on Cyber Monday.
Online services and smartphone apps were also affected, although the bank last night said that normal service had resumed and the systems issues resolved.
There was one report of a man unable to pay by card at a restaurant and then unable to withdraw cash to settle his bill, leaving him embarrassed and having to promise to come back to pay.
In an interview with the Belfast Telegraph following the third computer meltdown since June 2012, Mr Hamilton said Ulster Bank had failed – once again – to communicate properly with its clients.
"It is far better to tell your customers you don't know what's happening or what's wrong or how long it's going to take to fix something, rather than leave them completely unclear about the situation," he said.
"It isn't good enough and I'm sure the bank would accept that their customers haven't been served well on this occasion – just as they haven't been on the previous two occasions as well.
"IT problems happen, but the fact is, that it's the third time this has happened, and it is causing reputational damage to the Ulster Bank.
"I'm informed that it isn't the same IT problem.
"I don't know if that is reassuring or not, but Ulster Bank needs to redouble whatever efforts they have made in terms of investment in their IT system, to make sure it is robust and not susceptible to problems like this.
"This will be the final straw for many customers."
Up to 750,000 customers from RBS, Natwest and Ulster Bank – the Royal Bank of Scotland group – were hit by the fiasco.
It follows a major technical glitch in June last year, which left Ulster Bank customers without proper access to their bank accounts for weeks.
As a result, the bank ended up paying out £18m to almost 300,000 people in compensation.
A hardware problem also meant customers were unable to access their accounts, make online transactions or use their bank cards for several hours in March.
Stephen Shortley, from Ulster Bank's branch banking services, said the technical issue was different from last year's.
"It's not linked to what happened previously, it's not linked to hacking, or Cyber Monday, but in terms of each individual scenario, we are getting at it and got it fixed by late last night," he said.
"We fixed it as quickly as we could last night and I just want to say it was unacceptable to put customers, again, in that position.
"The detailed diagnostic work begins today, so we haven't got the final details, but what we can say is what went wrong last night is now fixed."
SDLP MLA and chair of the Stormont Enterprise committee, Patsy McGlone, said that Ulster Bank "has serious questions to answer".
"Last July, the bank assured the Enterprise Committee a detailed investigation would be carried out into the issue, which went on for weeks, but we're still trying to get to the bottom of it and waiting on a report from the Financial Conduct Agency," he said.
"If they have produced a report, we haven't seen it."
Sarah Mulgrew (23) was left mortified when her card was declined as she tried to pay for dinner on a night out with friends.
She said: "My card was declined twice, in front of a busy bar, so it was very embarrassing.
"I said I knew there was money in my account – I had checked it earlier. I was panicked, angry and very embarrassed.
"I walked up to a cash point, checked my balance and it didn't work, and I tried to login online, but nothing.
"This is the third time I've had an issue with the bank. I was affected the last time there was a computer problem, and received compensation of £275 for another issue just recently.
"As soon as I get this sorted – that's it. I will be getting out as soon as I can. But I want compensated. It's absolutely ridiculous."
Craig McMaster (23) and his girlfriend Megan Stinton (24) were left fuming as they tried to pay for a £200 Christmas shop.
"We had bought stuff for nieces and nephews and food and drink but when we went to pay for it, my card was declined and so was my partner Megan's. We weren't able to withdraw cash or transfer money from the Ulster Bank app to different accounts.
"I am furious. We were affected last time and it's an absolute joke.
"The cashier was looking at us as though we had no money and we were on the rampage. We had to leave everything back – everything. They left us with a receipt and said if we wanted to come back in three hours we could but we knew we couldn't because we had no access to funds.
"We have no loose cash to get by on – we withdraw cash daily. We both use the train to get to work and we don't know how we are going to do that."
In response to Ulster Bank's help services Craig said he was disappointed. "Ulster Bank's help? What help? They haven't done anything. They tweeted saying we are aware of technical issues, that doesn't help us, the ones who can't spend or access money."
June 19, 2012: A software update that went wrong led to a computer meltdown that left millions of Ulster Bank customers unable to withdraw cash or view an up-to-date account balance.
March 6, 2013: Customers unable to access their accounts or withdraw money.
March 28, 2013: Customers unable to use mobile banking for more than four hours
December 2, 2013: Customers unable to access hole in the wall machines, use cards or access online banking.
Will Ulster Bank customers switch following glitch?
BY CLAIRE WILLIAMSON
Thousands of account holders had their credit and debit cards declined at shop checkouts, petrol stations and restaurants across Northern Ireland on Monday.
Among them was 27-year-old Jenny Brennan, who told the Belfast Telegraph she now has little faith in Ulster Bank after her card was declined as she reached the check-out in Ikea with a trolley full of flat-packs.
She told the Belfast Telegraph: "We had a whole trolley full of stuff and we had to abandon it in the store, which was very embarrassing. I had £25 in cash to get our weekly shop.
"I have very little faith in Ulster Bank, as this is the second time in two years I've been affected.
"I think it is a definite possibility that I would consider moving now."
Ulster Bank said yesterday that that the IT meltdown had been resolved and systems were now working normally. But some still could not access their money.
Jane Lindsay said: "Just had two Ulster Bank card payments declined again today, there is definitely money in my account. Are there more problems?"
Another customer said: "Money left my account four times last night, when will I be getting it back?"
In September, the banking industry made it easier to switch accounts, promising the transfer could be done in a week, rather than a month.
Now, fears of another system crash have led some Ulster Bank customers to consider changing banks.
Paddy Byrne posted: "I gave Ulster Bank the benefit of the doubt the last time they made a hash of their accounts. Now it's time to leave!"
Michael Wright said: "Bye bye Ulster Bank. Personal and business accounts being pulled tomorrow. Stranded in Edinburgh last night without access to accounts. Not happy."
"So last night's fiasco broke me. Major stress with a hungry two-year-old. Time to find a new bank. No more Ulster Bank," Angie James said.
While Liam O'Connell added: "Can I pay for my purchases with Ulster Bank apologies, since they won't let me have my own money. Joke. Moving."
What the bank bosses said...
In a statement, Ulster Bank said: "The systems issues that affected our customers have now been resolved and all our services are now back working normally. We would like to apologise to our customers.
"If anyone has been left out of pocket as a result of these systems problems, we will put this right. If any customer is experiencing issues, they should get in touch with our call centres or come into our branches, where our staff will be ready to help."
Meanwhile, RSB Group CEO, Ross McEwan, said: "Monday night's systems failure was unacceptable. It was a busy shopping day and far too many of our customers were let down, unable to make purchases and withdraw cash.
"For decades, RBS failed to invest properly in its systems. We need to put our customers' needs at the centre of all we do. It will take time, but we are investing heavily in building IT systems our customers can rely on."
RBS bankers to pocket £500m bonuses
It may be at the centre of another humiliating public relations disaster, but Royal Bank of Scotland's bankers needn't worry –they are expected to rake in bonuses of £500m this year.
In most industries, bonuses are linked to good performance, although RBS bankers will still enjoy their half a billion pound payout after a steady stream of scandals.
RBS – which owns the Ulster Bank – was on the verge of collapse in 2008 when it was handed a government bailout of £45bn.
Yet last year, it paid out £607m in bonuses – despite losses of £5.2bn and still being 82%-owned by the taxpayer.
The payout would have been higher, but the bank had to set aside £300m to help pay its fines related to the Libor rate-rigging scandal.
Last month, RBS group chairman Sir Philip Hampton described how he felt like he was having an "out of body experience" when one banker protested that his £4m pay deal was unfair.
'Libor' is the rate the banks lend to each other, and after manipulating it for profit, RBS was fined £390m. Most recently, RBS has faced severe criticism after it was accused of running small businesses into the ground for its own benefit. The bank decided to press ahead with bonuses regardless.
A unit of the bank supposed to help turn around small businesses was alleged to have driven some to collapse in order to buy back their assets at rock bottom prices.
RBS said there was "no evidence" of the claims.
Bank of England governor Mark Carney said the reports were "deeply troubling" and ought to be pursued "to full extent of the law".