The board of TSB lacked “common sense” and shifted customers to a new IT platform before it had been fully tested, an independent report into the bank’s 2018 meltdown has found.

The report, commissioned by the bank and carried out by law firm Slaughter and May, said the board failed to ask its contractor key questions ahead of the launch, which resulted in nearly 1.9 million customers being locked out of their accounts.

TSB’s former chief executive Paul Pester, who was ousted months after the debacle, criticised the report’s “scattergun” approach, but said its findings showed TSB’s Spanish parent company Sabadell had “cut corners” with critical IT testing.

“If these findings are right, Sabis [Sabadell’s in-house IT arm] 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,” Pester said.

“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.”

However, the report notes that the board failed to ask key questions after facing months of delays to the rollout, which resulted in the creation of a new migration programme plan dubbed the “defender plan.”

“While the TSB board asked a number of pertinent questions regarding the defender plan, there were certain additional, commonsense challenges that the TSB board did not put to the executive (including why it was reasonable to expect that TSB would be ‘migration ready’ only four months later than originally planned, when certain work streams were as much as seven months behind schedule).”

The migration plans were meant to finally separate TSB from the IT systems owned by its former parent Lloyds Banking Group. But the report claimed its leadership failed to learn any lessons from the programme’s initial stumbles.

The report said the new plan had “presented an opportunity for TSB to take a realistic view of the status of the programme and learn from the programme’s first 18 months. This opportunity was missed … there was little attempt to investigate the technical causes of the delays that had been faced to date.”

In particular, TSB is accused of failing to “adequately” assess whether Sabis was capable of delivering the new plan.

As a result, “TSB went live with a platform that was, in the event, neither stable nor complete.”

Commenting on the report, TSB chairman Richard Meddings apologised for the disruption but said he did not agree with all of its findings. He said technical issues were at the heart of the failure, and that the two data centres were not build to the same specifications.

“When we commissioned Slaughter and May to carry out this review, we specifically asked for a fully independent and thorough inquiry. Although the report doesn’t paint the full picture of migration, the Board were absolutely clear that we wanted to be transparent and learn fully from those aspects which went wrong,” Meddings.

Millions of customers suffered severe disruptions online and in branches in the weeks that followed the botched launch. Pester resigned as TSB chief executive following intense criticism from regulators and MPs. He gave up his 2018 bonus related to the migration but was still granted just over £800,000 in severance pay.

He was later replaced by former CYBG executive Debbie Crosbie, who took the reins in May this year.

Sabadell announced earlier this year that it had resolved all of the 184,000 complaints lodged over the IT problems. TSB has paid £366m to cover the costs of the IT meltdown, about £130m of which was used to compensate customers. That total also includes the bill for the Slaughter and May report, which cost up to £25m.

Investigations by City regulators the Financial Conduct Authority and Prudential Regulation Authority are ongoing and they have the power to levy an unlimited fine on TSB.

The FCA said: “The responsibility for decision-making in the lead-up to any migration like this lies with the firm and we expect sufficient oversight and engagement from senior management before any migration occurs.”

It added: “Our investigation with the PRA into the events at TSB is wider in scope than the Slaughter and May review. We are well advanced, but as is our usual practice we can’t confirm precise timing while the investigation is still under way.”

The report has been released one week before Crosbie is due to unveil her new strategy for the lender.

The bank is in a race to cut about £100m of costs and has put its 544-branch network under scrutiny. Crosbie could announce up to 100 branch closures, putting a further 400 jobs at risk.