TSB’s computer meltdown pushed the bank into a £105.4m loss last year, from a £162.7m profit in 2017.
The bank estimates that its attempt to move its IT system resulted in £330.2m in costs.
About 80,000 customers switched their bank account away from TSB in 2018 – 30,000 more than 2017 – after some people went weeks without services.
There was also the cost of customer compensation and fraud. TSB said it had recovered £153m from IT provider Sabis.
TSB said it had resolved about 90% (181,000) of the 204,000 customer complaints received since the IT chaos.
And in recognition of the extra work for its employees, the bank said it had awarded staff – excluding executives – £1,500 each in December.
Richard Meddings, TSB executive chairman, said: “Last year was TSB’s most challenging year. But we enter 2019 with renewed ambition to re-emerge as the leading challenger bank in the UK – firmly on the side of the customer.”
Up to 1.9 million digital and mobile banking customers were left unable to access their accounts in April last year after work to transfer its IT system failed.
The tech troubles were triggered by a migration of customer data from the IT system of its former owner Lloyds to a new one managed by the bank’s Spanish owner, Sabadell.
Chief executive Paul Pester stepped down in the wake of the fiasco.
The bank announced his replacement, former CYBG chief operating officer Ms Debbie Crosbie in November. She will join the business in the spring.