Video Report by Business EditorJoel Hills
The boss of TSB has been scolded by MPs for stating that customers were "shouting into the void" when complaining about the IT meltdown on social media.
TSB boss Paul Pester admitted to the Treasury Select Committee the company had received more than 40,000 complaints over the fiasco.
But he added that customers should use TSB’s online complaint form to get in touch, rather than trying to phone or using social media to report problems.
The committee's chair Nicky Morgan hit back at the comment saying: "What we are hearing this afternoon is the most staggering example of a chief executive who seems unwilling to realise the scale of the problem that is being faced."
The banker faced a grilling from MPs on Wednesday after thousands were left without access to their accounts.
Mr Pester said: "Clearly we have caused some serious disruption to our customers over the last 10 days and for that we are truly sorry."
TSB had intended to complete a major IT system upgrade on the weekend of April 21 and 22, although problems persisted.
Delays meant customers were left unable to access their money online, while some reported being able to access other customers’ accounts.
"The issues that we are seeing, for which I apologise again to our customers, is that the platform's ability to support sufficient customers accessing our website and mobile app simultaneously was an issue for us Monday, Tuesday, Wednesday last week," he said.
The banker added that around 95% of customers were now having success accessing the bank's internet and mobile services, adding: "We did not see those figures soon after the migration and as I say that led to an unacceptable level of performance to our customers."
Mr Pester said: "As the chief executive of the bank I absolutely take full responsibility for what has happened with TSB."
Asked if he had failed to ensure the basic duty of a bank - to let customers put money in, take money out and keep money safe and secure - Mr Pester said: "I feel we have failed in delivering the level of service we need to deliver to our customers, clearly.
"TSB was built to bring local banking to Britain. TSB was built to reconnect banks to local communities."
He continued: "I'm deeply sad that the work we have done over the last five years in building TSB as a different sort of bank has been so damaged over the last 10 days.
"But we will come back as TSB.
"TSB will come back and bring the competition it needs to bring to UK banking."
Timeline of TSB IT meltdown
Asked why his customers should ever trust him personally again, Mr Pester said: "They should trust me because I will ensure that we bring TSB out of the problems we're in.
"I've given my cast-iron guarantee to customers that they will not be out of pocket as a consequence of these problems.
"And our customers know that I have their interests at heart."
Mr Pester said 40,000 complaints had been received, whereas around 3,000 would normally be expected over a 10-day period.
"Those 40,000 complaints are being investigated. Those 40,000 complaints will be put right."
Mr Pester, told by the committee that two members of staff were unable to log on to their TSB mobile app while the session was going on, replied: "It's nice to know there are so many customers in the room. Thank you very much for using TSB."
Asked if his bonus was in any way contingent on the migration happening in April, Mr Pester replied: "Not in April, no. There was a reward for completing migration but it's not linked to timing."
He said all the rehearsals and tests beforehand "gave us belief that we were ready for the migration".
"The thing we're struggling with is what happened after the migration, which clearly showed that those tests were misleading us.
"We have to get to the bottom of that."
Quizzed over reports from customers that they are waiting "huge amounts of time" to get through to customer services via the phone, he said: "Just to be clear I am not defending our telephone service.
"It is very, very poor. It is unacceptable.
"Customers are having to wait 30 minutes to get through to us.
"Typically a call to a TSB call centre lasts for four minutes. Our current calls last for closer to 15 minutes."
Mr Pester also admitted he couldn't give a deadline for when the "shambles" will be over.
"I can't give you a fixed date, if I do it's likely to be unfair to our customers."