TSB saw over 20,000 customers ditch the Spanish-owned bank in the wake of an IT meltdown earlier this year, according to figures released by the payments operator BACS.
Data from the body, which operates the seven-day current account switch service (CASS), showed 21,790 people left TSB in the second quarter of this year between April and June, with a net gain/loss figure of -16,641.
As many as 1.9mln TSB customers were left unable to access their own money in April, after the bank tried to introduce a new computer system, resulting in an enquiry from the City regulator and the resignation of the bank's chief executive, Paul Pester in September.
The Financial Conduct Authority is still considering whether to fine the bank after it was judged to have misled Parliament over the issue.
TSB suffered further computer problems in September which could also bring further switching of customer accounts elsewhere.
The next biggest customer loser in the quarter, according to the BACS data, was Royal Bank of Scotland Group PLC (LON:RBS) which saw 14,445 customers switch their accounts elsewhere between April and June, with a net gain/loss of -12,362.
Overall, BACS said a total of 693,422 switches have taken place since the start of this year with 193,621 completing in the last quarter alone – July to September 2018.
Between 1 October 2017 and 30 September 2018, 941,122 switches were completed – up 5% on the previous 12 months – the banking body added.
Since CASS was launched in 2013, BAC said over 5.1mln bank current account successful switches have taken place.