The Financial Ombudsman Service (FOS) upheld 93% of complaints against TSB in 2018 following its IT meltdown in April.
Figures released by FOS show 4,849 customers had contacted them after failing to get satisfaction by complaining direct to the bank.
Journalist Laura Miller revealed the uphold rate for the complaints is three times higher than for other comparable banks, adding: “The startlingly high uphold rate suggests in many cases TSB could have resolved the complaint and paid out compensation to customers much earlier.
“More than 1,300 customers lost money through fraud attacks and 370 people were wrongly told they had died.”
The bank’s problems began when a changeover in computer servers went disastrously wrong.
Problems with the transfer of data from the servers of its former owners Lloyds to its new owners Sabadell and an estimated 1.9 million customers were locked out of their accounts for several weeks.
The true scale of the problem was revealed in February when TSB reported it had slipped £150.4 million into the red as opposed to a profit of £162.7 million in 2017.
It admitted the episode had cost it £330.2 million which included customer compensation, additional resources, fraud and foregone income. Approximately 80,000 customers switched their accounts elsewhere.