A report by the law firm Slaughter and May said that a new computer system's test failures are the cause of problems left to 1.9 million TSB customers without access to online banking.
In April 2018, the bank began moving customers to a new system, but some could not use it for several weeks.
TSB, part of the Spanish banking group Sabadell, established the Sabis system, an internal IT provider.
The report stated that the tests performed on the new system were not performed correctly.
Tests were conducted only in an offline environment, but not in a live system.
He said TSB acknowledged that tests were conducted on both systems and could identify problems affecting customers before they occurred.
"We concluded that the new platform is not ready to support TSB's full customer base and Sabis is not ready to use the new platform." Said.
TSB President Richard Meddings said: "Slaughter and May's report reveals a number of findings about some aspects that they believe are different aspects of planning and preparing for migration.
Inda In the light of experienced glitch customers, TSB has made significant changes to ensure bank restructuring – including leadership and management structures, as well as the decision to directly control IT operations.
"Importantly, TSB has long compensated all eligible customers affected by the outage."
Paul Pester, the former general manager of TSB who quit his job a few months after the incident, said, sa If these findings are true, Sabis threw the dice by conducting tests in only one of TSB's two new data centers and hiding the decision from me. and the rest of the TSB board.
"The report explains that this makes it impossible for TSB's board of directors to anticipate serious problems experienced by many customers who cannot access their accounts.
“Obviously, if we were aware of Sabis's shortcuts in the testing program, TSB was installed and then we never put pressure on the new system,” he said.