Testing failures of a new computer system were the cause of problems that left up to 1.9 million TSB customers without online banking access, a report by law firm Slaughter and May has said.
In April 2018, the bank started moving customers to a new system, but some could not use it for several weeks.
TSB is part of the Spanish banking group Sabadell and its in-house IT provider Sabis built the system.
The report said tests on the new system were not carried out properly.
It found that the tests only took place in an offline environment but not also in a live system.
It said that TSB accepted that had tests been run across both systems, it might have been able to identify the issues which affected customers before they happened.
The report said: “We have concluded that the new platform was not ready to support TSB’s full customer base and Sabis was not ready to operate the new platform.”
TSB executive chairman Richard Meddings said: “Slaughter and May’s report sets out a number of findings on aspects of the planning and preparation for migration which they believe could have been done differently.
“In light of the disruption customers experienced, TSB has made important changes to enable the bank to rebuild – including to leadership and management structures, as well as the decision to take direct control of its IT operations.
“Importantly, TSB has long since compensated every eligible customer who was impacted by the disruption.”
TSB’s former chief executive Paul Pester, who quit his job a few months after the incident, said: “If these findings are right, Sabis rolled the dice by running tests on only one of TSB’s two new data centres and this decision was kept from me and the rest of the TSB board.
“The report explains that this made it impossible for the TSB board to anticipate the serious problems experienced by many customers who could not access their accounts.
“Obviously, if we had been aware of Sabis’s shortcuts in the testing programme, the TSB board and I would never have pressed ahead with switching to the new system at that time.”