The report found that testing only took place offline and not in a live environment, hampering the ability to spot potential problems.
Up to 1.9 million customers were affected by the problems which left them unable to manage their accounts and opened them up to fraud.
£330m was paid by TSB in compensation, fraud and other expenses as a result of the IT failure.
TSB partnered with IT provider Sabis to switch over to a new IT system that was four years in the planning, with the migration taking place in April 2018.
When TSB separated from Lloyds Banking Group in 2013, they continued to use a legacy IT platform. The final switchover to the new system transferred every customer account, but then the problems started.
Although the system was tested with over 1,600 employees, testing in a live environment didn't take place until all the data had been migrated.
As well as this, the report found only 800 system defects were conveyed to the board when over 2,000 had been uncovered.
Failings by both TSB and Sabis were highlighted, with the bank criticised for setting unnecessary time constraints on the project and the IT provider criticised for being unprepared to operate the new platform.
TSB have apologised for the service disruption and commissioned the independent review to reassure customers they were committed to learning from their failures.
The TSB system crash was one of the most severe IT failures of recent years, impacting 1.9 million customers, causing untold stress and contributing to an increase in complaints to the Financial Conduct Authority (FCA) in the first half of 2018.
While all impacted customers have been compensated for the disruption and TSB seem to have recovered financially judging by their most recent results, the incident is indicative of the problems facing companies in a world reliant on IT.
A bank like TSB obviously relies on the integrity of their systems, and this was a blow to their reputation, but firms in other sectors like Plusnet have switched to new billing systems and experienced difficulties afterwards.
The Treasury Committee recently conducted an inquiry into the frequent IT crashes affecting the financial sector, concluding prolonged failures were unacceptable and must be addressed by the regulators.
At the time, they also criticised the report into TSB's failures for taking too long to deliver, with over 18 months having passed since the incident.
Issuing the independent report now means TSB's failures are back in the headlines over a year and a half after the IT failure itself.
That said, the bank can legitimately argue they have made changes to their processes by taking direct control of their IT operations, and at least publishing the report guarantees full transparency for both the sector and customers.
TSB have 3.8 million customer current accounts according to their H1 2019 results and made the news in April 2019 by being the first bank to launch a Fraud Refund Guarantee ahead of industry-wide voluntary protections coming into force.
This guarantee was designed to cover customers against the risks of third-party fraud transactions which banks were previously able to claim were authorised.
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