The Financial Conduct Authority (FCA) and Bank of England's Prudential Regulation Authority (PRA) identified "widespread" deficiencies in the planning and execution of a customer data migration in April that year.

TSB - which was spun out of Lloyds Banking Group, listed on the stock exchange in 2014 and acquired by Sabadell the following year - had been attempting to shift the data from Lloyds' systems to a new one designed by its Spanish owner.

But the new platform suffered technical difficulties after the transfer, resulting in significant disruption to the continuity of banking services including branch, telephone, online and mobile banking.

"All of TSB's branches and a significant proportion of its 5.2 million customers were affected by the initial issues", the FCA said in a statement.

"Some customers continued to be affected by some issues and it took until December 2018 for TSB to return to business as usual. TSB has paid £32.7m in redress to customers who suffered detriment."

The saga proved a PR disaster for the bank and prompted its-then chief executive, Paul Pester, to resign.

He had admitted at the height of the crisis that the lender was "on its knees".

The regulators said: "TSB's IT migration programme was an ambitious and complex IT change management programme carrying a high level of operational risk.

"Its success was critical to TSB's ability to provide continuity of critical functions and safety and soundness.

"However, the regulators' found that TSB failed to organise and control the IT migration programme adequately, and it failed to manage the operational risks arising from its IT outsourcing arrangements with its critical third-party supplier."

TSB was fined £29.75m by the FCA and £18.9m by the PRA.

Mark Steward, FCA executive director of enforcement and market oversight, said: "The failings in this case were widespread and serious which had a real impact on the day-to-day lives of a significant proportion of TSB's customers, including those who were vulnerable."

TSB's current chief executive, Robin Bulloch, said of the settlement: "We'd like to apologise again to TSB customers who were impacted by issues following the technology migration in 2018.

"We worked hard to put things right for customers then and have since transformed our business.

"Over the past four years, we have harnessed our technology to deliver new products and better services for TSB customers."

Sabadell said in a statement that the settlement would be accounted for by TSB in the fourth quarter.