The crisis happened as the bank began to migrate current accounts to a new core banking platform from Banco Sabadell. The independent inquiry into the subsequent chaos calls out the TSB board for failing to address "common sense challenges" about aspects of the IT migration process.

Published today, the damning report into the incident by law firm Slaughter & May places the blame for the shambolic migration on a decision not to test a backup data centre, and lays bare the culpability of senior staff at Banco Sabadell.

Sabis, the IT servicing arm of Banco Sabadell, had recommended testing only one of the data centres in order to avoid interrupting ATM services for TSB customers. The failure to conduct rigorous testing made it impossible to identify the problems causing the downtime when the system went dark, the report suggests.

Carlos Abarca, the bank's chief information officer at the time, has also come in for stinging criticism, for allegedly failing to tell Board members about the shortcomings before the button was pushed for the migration to commence. Under pressure from Banco Sabadell to meet the migration deadline, the post-mortem suggests that he made an ill-judged assessment of TSB's readiness to proceed with the move.

The board of TSB has pushed back on a number of the report's finding.

In a statement, the bank says: "There are aspects of Slaughter and May’s report with which the Board does not agree. In particular, a key cause of the extent of disruption was that the two data centres, built to support the new platform were, in certain areas, configured inconsistently despite having been specified to be identical. Additional issues around coding and capacity also arose. These technical issues were then compounded by the high volume of customer enquiries as public concern increased - enquiries which exceeded the contingency resources already in place.

"The information the TSB Board considered at the time that they made the decision to proceed with migration did not suggest that customers would be impacted in this way.

"The decision to proceed with the main migration event in April 2018 followed a long period of testing, with reports to the Board signalling readiness, and took account of the successful performance of those parts of the bank which had already moved to the new platform.

"Additionally, the Board carefully considered comprehensive expert scrutiny, attestations and assurances from the Executives and third-party suppliers including SABIS, and feedback from technical, third party, and regulatory bodies throughout. The overall governance of the programme had also been considered by external experts prior to the migration and assessed to be rigorous and appropriate for a programme of this scale and complexity."

TSB has already had to pay close to £370m in post-migration charges as a result of the meltdown. Further fines from regulatory bodies upon publication of the report could push this total higher.

The release of the report comes a week before a strategy update from Debbie Crosby, the CEO who replaced fall guy Paul Pester in the role, which is likely to lead to closure of up to a hundred branches and the loss of 400 jobs as the bank seeks to make up to £100 million in cost savings.