An IT meltdown which locked Royal Bank of Scotland customers out of their accounts last summer is being investigated by the City regulator - potentially triggering a big fine for the part-nationalised bank.
The Financial Conduct Authority (FCA) has started an enforcement investigation after the fiasco in June and July, which saw payments go awry, wages appear to go missing and home purchases and holidays interrupted.
The glitch affected up to 17 million customers of RBS, NatWest and Ulster Bank.
Public confirmation of a probe prior to its completion is unprecedented for the regulator, which took over from the Financial Services Authority at the start of the month.
The 81% state-owned bank could face a fine, censure or both.
RBS has already taken a £175 million hit to cover costs and compensation relating to the calamity, while the incident prompted chief executive Stephen Hester not to take his 2012 bonus.
The FCA said: "The Financial Conduct Authority has started to conduct an enforcement investigation into the IT failures at RBS which affected the bank's customers in June and July 2012.
"The FCA will reach its conclusions in due course and will decide whether or not enforcement action should follow that investigation."
It is rare for the regulator to launch a formal probe into a firm over an IT failure. The overwhelming public interest in the case is believed to have prompted the FCA to stray from its usual remit of investigating market abuse, mis-selling and data security lapses.
RBS did not give details on what went wrong in the summer, but it reportedly followed an attempt to install a software update on RBS's payment processing system, which was then corrupted. It meant account balances were not updated properly overnight and credit and debit balances failed to show up as quickly as they should.