Royal Bank of Scotland (RBS) has been fined £56m by regulators after an IT problem in 2012 locked over six million of its customers out of their bank accounts for days.
The bank was fined £42m by the Financial Conduct Authority (FCA) and £14m by the Prudential Regulation Authority.
The outage was caused by a software error during a systems upgrade two years ago, and affected NatWest and Ulster Bank customers as well as those from RBS.
Millions of customers were unable to get balance information from ATMs, make mortgage payments or withdraw cash in foreign countries.
The bank also applied incorrect credit and debit interest payments to customers, and provided inaccurate bank statements.
The FCA placed the blame firmly at RBS' feet, accusing the company of failing to install systems resilient enough to withstand or minimise the risk of IT failure.
Tracey McDermott, director of enforcement and financial crime at the FCA, said: "The problems arose due to failures at many levels within the RBS Group to identify and manage the risks which can flow from disruptive IT incidents. The result was that RBS customers were left exposed to these risks."
RBS earmarked £175m to reimburse customers who had suffered financial losses, and pledged to invest more than £1bn in its digital capabilities and IT over the next three years.
The fine far outstrips the £2.75m handed to Ulster Bank after a 28-day IT meltdown in 2012.
McDermott highlighted the importance of a robust IT system for banks and financial services companies.
"We expect all firms to focus on how they ensure that they can meet the requirements of their customers when looking at their IT strategies and policies," she said.
IT failure is not the only technology challenge for which banks need to prepare. London's police chief warned this week about the global implications of a cyber attack against financial institutions in the UK and US.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago