This appears to be an IT failure comparable to the 2012 and 2015 IT failures at the Royal Bank of Scotland. These involved multi-day / multi-week full/partial outages, and multi-million pound fines.
Background: TSB is a UK bank. They had a planned 50-hour maintenance window over the weekend (Fri 16:00 - Sun 18:00), to move away from Lloyds's systems (following the "un-merger" in 2013-2014). This went wrong, in what is likely to become a textbook example of an IT failure.
FT: https://www.ft.com/content/9178ef60-46d9-11e8-8ee8-cae73aab7...
Background: TSB is a UK bank. They had a planned 50-hour maintenance window over the weekend (Fri 16:00 - Sun 18:00), to move away from Lloyds's systems (following the "un-merger" in 2013-2014). This went wrong, in what is likely to become a textbook example of an IT failure.