ULSTER Bank has been hit with a €3.5 million (£2.75m) fine for the IT meltdown two summers ago which cost it nearly £50 million to put right.
But that sanction from the Central Bank of Ireland may be just the tip of the iceberg because the British regulatory watchdogs are expected to follow with an even heftier fine this side of Christmas.
The Financial Conduct Authority is also probing the failure at the taxpayer-backed bank, with sources putting the scale of the likely fine to be imposed on Ulster Bank's parent company RBS at "several tens of millions of pounds", which would rank it among the largest ever handed out by the City regulator for offences unrelated to the manipulation of financial markets.
The July 2012 glitch - which stemmed from a computer system upgrade its at parent company RBS - left 600,000 Ulster Bank customers across Ireland uncertain that direct debits were being paid, unable to check their balance, receive their salary or access cash over the holiday period.
Northern Ireland was the worst-hit region across the RBS network, where the bank was forced to redeploy more than 100 staff and extend its opening hours to Saturdays and Sundays to deal with the backlog.
Indeed, it was the best part of four weeks before things returned to normal, and Ulster Bank was forced to offer compensation for out-of-pocket expenses, waive interest and charges, and give current account holders in the north a £20 one-off pay-out.
The meltdown was described by the Central Bank of Ireland as "an unprecedented disruption", and its €3.5m fine - which has been accepted by Ulster Bank - is the highest it has ever imposed.
The Central Bank said Ulster Bank had failed to put in place "robust governance arrangements" to prevent and deal with the IT failure. It said the technological breakdown not only caused "widespread and significant loss" to the bank's customers, but also threatened confidence in the banking retail sector.
The Central Bank's director of enforcement, Derville Rowland, said: "As the provision of financial services to customers represents the core business function of the firm, the major breakdown in the firm's provision of these services as a result of IT failings is completely unacceptable."
She said the size of the fine indicated the scale of the disruption.
"It reflects the seriousness with which the Central Bank views the failings of the firm and the Central Bank's determination to ensure that customers have access to core banking services without disruption," she said.
To date Ulster Bank has paid out more than £46 million (€59 million) to customers as part of a redress scheme ordered by the Central Bank.