A failed software upgrade - which snowballed into 700,000 Ulster Bank customers being unable to use their bank accounts for three weeks - led to the biggest ever fine handed down by the regulator.
The Financial Conduct Authority (FCA) and the Bank of England's Prudential Regulator Authority (PRA) slapped the Royal Bank of Scotland (RBS), which owns Ulster Bank, with a fine of £56m for the IT meltdown in June and July 2012.
The banking group, which also includes NatWest, has already paid £70.3m in compensation to its customers because of the problem. A further £460,000 has been paid to those who were not customers but were still affected.
Earlier this month, Ulster Bank was fined €3.5m by the Republic of Ireland's financial regulators, the Central Bank of Ireland (CBI), for the effect it had on the bank's customers south of the border.
One in 10 people in the UK were affected by the glitch, which was reflected in the size of the fine.
In total, 6.5m people were hit, of which 700,000 were Ulster Bank customers.
Philip Hampton, chairman of RBS, said: "Our IT failure in the summer of 2012 revealed unacceptable weaknesses in our systems and caused significant stress for many of our customers. As I did back then, I again want to apologise to all customers in the UK and Ireland."
The IT glitch meant that customers were unable to access accurate account information at ATMs, access their online banking, make mortgage payments, and some businesses were unable to pay staff.
The problems, which started on June 20, 2012, continued for three weeks for Ulster Bank customers, while RBS and Natwest's customers issues were resolved more than a week earlier.
The meltdown was caused by a failed upgrade to software used to update the bank's transactions.
The regulator said there were not sufficient systems in place to manage their exposure to IT risks.
Aodhan O'Donnell, interim chief executive of the Consumer Council in Northern Ireland, said: "Ulster Bank accounts were the last to be restored, meaning customers here had to wait an unacceptable length of time, with many unable to use their account for up to one month.
"What started as a technical failure quickly escalated into a payments crisis and a communications disaster, with the lack of clear information compounding consumer anxiety and distress."
Tracey McDermott, director of enforcement and financial crime at the FCA, said modern banking depended on "effective, reliable and resilient IT systems". "The banks' failures meant millions of customers were unable to carry out the banking transactions which keep businesses and people's everyday lives moving."
In 2013, RBS announced an increased investment of £750m for three years on top of its yearly IT spend, to improve its systems.
Of the £56m fine, £42m is from the FCA and £14m from the PRA - the first fine the latter has handed down.
While RBS said no customers left the bank as a direct result of the meltdown, Payments Council figures showed all three banks in the group lost current account customers in the first three months of the year. Ulster Bank had a net loss of 1,535 customers between January and April, while Natwest had a net loss of 18,258, and RBS a net loss of 9,971.
Yesterday's fine of £56m for Royal Bank of Scotland (RBS) by the financial regulators was the latest in a long line of fines for the part taxpayer-owned bank. Earlier this month, it was fined £402m by US and UK regulators for its role in the rigging of foreign exchange markets. It was also fined £14.7m in August for serious failings with its sales of mortgages.