Ulster Bank has admitted it has no idea of the numbers affected by its latest computer glitch – provoking more fury among already outraged customers.
Their anger has been exacerbated as no explanation has yet been given for the problem that saw cash machine withdrawals being debited twice from some accounts.
It is the fifth time the bank has been beset with technical issues since its entire IT system broke down in June 2012, prompting the worst technical crisis in modern banking.
Ulster Bank has promised the cash will be reimbursed by tonight and apologised for any inconvenience caused by recent IT issues.
However, many customers have said they are appalled that the banking giant didn't admit the problem until after it claimed it had been fixed.
Mark Hood, a 38-year-old chef from Belfast, told this newspaper that he had been debited £120 after making a £30 cash withdrawal.
"I've had several problems with Ulster Bank over the past couple of years and this really is the last straw for me," he said. "I withdrew £30 from a cash machine on April 22 but, according to my account, I took out £120 in total because I've been debited £30 twice, £20 twice and £10 twice.
"It's an absolute joke. I've been a customer for 38 years but enough is enough. I'm furious that they didn't warn us there were problems; we had to find out for ourselves the hard way."
Another customer, Matthew Holland (21), who is from Belfast but is currently studying finance at university in Newcastle, said that twice the amount he withdrew had been debited.
"I withdrew £30 to do some shopping on Easter Monday and then I noticed that £60 had been taken from my account," he said.
"I feel sorry for the employees who have to deal with these ongoing issues, but I'm definitely going to switch banks now."
In a statement issued yesterday Ulster Bank admitted there had been problems for a 24-hour period.
"We are aware of an issue regarding duplicate ATM transactions involving some Ulster Bank customers between 4.15pm on Easter Monday and 4.15pm on Tuesday afternoon," its statement read. "We are working to correct this for affected customers and restore money to accounts tonight.
"Customers will not be left out of pocket as a result and can contact us if they require further assistance. We apologise for any inconvenience caused."
Bank accounts were affected on Monday and Tuesday this week while many people were off work for the Easter holiday.
The problems also occurred close to the end of the month when money can be scarce for many people waiting to get paid.
Some people vented their anger on Twitter yesterday morning.
One man posted: "Debited twice for 1 cash withdrawal on Monday. Happening to plenty of others too! Explanation and resolution? Joke!"
Another woman wrote: "@UlsterBank_Help I've a direct debit coming out and you took the money that wasn't yours.. What happens now though?"
In 2012 a serious IT failure at the bank's parent company RBS caused widespread chaos for many customers who were unable to access their funds.
At the start of this month union official Seamas Shiels, from the Irish Bank Officials Association, told Stormont's enterprise committee that he didn't think "any of the banks in Ireland have a genuinely stable IT platform". But Ulster Bank's chief executive Jim Brown told MLAs that the IT software problem that led to the 2012 systems meltdown had been fixed following massive investment by RBS.
He said that £450m has been spent "specifically on improving the resilience of systems across RBS and Ulster Bank... on top of the £2bn spent every year on IT".
Two years of technical woes
JUNE 19, 2012
The automated procedure that processes payments in batches into Ulster Bank accounts goes wrong. About 100,000 customers are affected by the disruption to salary transfers, direct debits and social welfare payments, causing a one-day delay in processing certain payments to bank accounts. Ulster Bank opens 78 of its branches until 7pm in cities and major towns in order to help customers access their funds.
JULY 2, 2012
Ulster Bank promises to carry out a full investigation into the the problem, overseen by independent experts, once the critical system recovery tasks are completed and pledges to continue liaising closely with regulators.
MARCH 7, 2013
Customers are unable to withdraw cash from ATMs or access their accounts online or by telephone after cards are declined. The bank says the problem, which lasts several hours, stemmed from a wider issue in Britain affecting Ulster Bank owner, RBS.
DECEMBER 3, 2013
On Cyber Monday, the biggest online shopping day of the year, Ulster Bank customers are unable to use their credit or debit cards or access their online accounts. Many are left at shop tills unable to complete transactions or pay bills.
DECEMBER 12, 2013
Customers have trouble accessing current transactions, wages aren't paid into accounts and other payments haven't been processed. The bank says the problems are due to a missing batch of credits.
APRIL 21 & 22, 2014
Customers report that cash machine withdrawals have been taken from their accounts twice. The problem lasts for a 24-hour period during Monday and Tuesday this week. The bank promises that customers will not be left out of pocket and apologises for any inconvenience caused.
Analysis: Loyal customers find their patience tested to the limit
BY PAUL GOSLING
No one should have been surprised by the latest IT problem afflicting Ulster Bank.
It is just the latest in a series of issues that have caused distress to Ulster Bank and its surprisingly loyal customers. The most notorious of these occurred in the summer of 2012 when thousands of people were locked out of their accounts for several weeks.
That IT meltdown has so far cost Ulster Bank and its largely taxpayer-owned parent RBS some £175m in compensation paid to customers. RBS will pay many millions more when the UK regulator, the Financial Conduct Authority, completes its investigation into the failure.
But while that crisis made the headlines, other less serious incidents had less attention. In March last year customers were unable to use cash machines or conduct online transactions for several hours. That same month some of the bank's own staff were paid late. Two months later the Ulster Bank's smart phone app failed for a while.
In December of last year – on the busiest pre-Christmas shopping day – card payments and cash withdrawals suddenly stopped functioning. A few days later hackers brought down the Ulster Bank and RBS online systems. Another week later Ulster Bank was unable to conduct some euro transactions.
The broader picture is clear: Ulster Bank has an IT system that is not fit for purpose. Despite allocating an extra £450m on top of the scheduled annual £2bn spend on IT across the RBS group, it is no simple matter to correct such a seriously defective system.
My understanding is that, for historical reasons, the Ulster Bank IT system is plugged into the NatWest system, which in turn is plugged into the RBS system. But, apparently, the different systems process transactions in ways that are not fully compatible. This explains much of the difficulties.
Until now Ulster Bank customers have been remarkably patient. There is a saying in banking that customers are more likely to get divorced than to move their current accounts. But that inertia factor may be about to change. It has never been so easy to move bank accounts and the process is usually trouble-free.
Until now Ulster Bank seems to have hoped that no one will notice when its systems hiccup. It should by now have got the message. Customers and the media find out quickly. Transparency and openness, with realistic assessments of how long it will take to put things right, just might save the day until that new IT system is up and ready. If not, Ulster Bank may not be able to take customer loyalty for granted.
Paul Gosling is a journalist specialising in personal finance