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The technology meltdown at TSB earlier this year has cost the bank £176.4m, pushing it into a half-year loss.
The bank said its performance was "significantly impacted" by the IT failure.
It reported a loss of £107.4m in the six months to 30 June compared with a profit of £108.3m last year.
In April, almost two million TSB customers lost access to online banking services after the bank bungled the introduction of a new IT system.
Around 26,000 customers closed their TSB account in the second quarter. However, 20,000 opened a new account making a net loss of 6,000 customers.
"I know how frustrated many customers have been by what's happened," said TSB chief executive Paul Pester.
"It was not acceptable, and was not the level of service that we pride ourselves onnor was it what our customers have come to expect from TSB."