(Sharecast News) - Nearly 22,000 current account customers quit under-fire bank TSB in the second quarter, according to data from payments processing body Bacs published on Wednesday.TSB, which endured a major IT meltdown earlier this year, saw 21,790 customers leave between April and June. During that time, it also gained 5,149 new current account customers, given a total net loss of 16,641.That was the highest loss of any of the UK high street banks. The next biggest were at NatWest and its parent Royal Bank of Scotland, which saw net losses of 12,053 and 12,362 respectively. The biggest net gain was made by Nationwide Building Society, which added 34,577 current account customers.TSB customers were left unable to access their money and without online banking services for weeks after the bank tried to introduce a new IT system in April. The botched upgrade also saw a surge in attacks by fraudsters and led to the resignation of chief executive Paul Pester after seven years at the helm.The Financial Conduct Authority is still considering whether to fine the bank after it was judged to have mis-led Parliament. The affair has already cost Spanish parent Banco Sabadell, which acquired TSB in 2015, around €200m.However, TSB - which has a total of 15.2m customers - insisted that it had attracted more than 20,000 new customers across all the accounts it offers during the period, with current accounts representing just one part of its customer base.