The introduction of a new IT system left customers struggling to make transactions and able to see their balances.
The Financial Conduct Authority (FCA) said to the bank's boss Paul Pester an "optimistic view" of services had given. He was appearing before MPs for a second time on the crisis.
FCA chief executive Andrew Bailey said: "We do not normally make this information public but we have given the level of public interest and, I want to be clear that we will be conducting on this work"
He added that the bank's poor communication may have hit customers' trust in banking transaction over the days passed.
Its five million customers were moved to a system run by its Spanish owner “Sabadell” six weeks ago. That’s the reason some TSB customers are still facing problems with the bank’s services.
In the midst of the crisis, Mr. Pester told MPs on the Treasury in a Committee that most TSB customers could carry out transactions as normal. In a letter, the FCA's Mr. Bailey said to the committee that view was optimistic and too positive.
"Greater concern would have to make a sense". Mr. Pester said he should have shared more details about the problems with MPs on the committee.
The customers were told by the bank that the vast majority of them could use their online accounts services but only half could do this.
Mr. Bailey wrote in a statement that "The FCA has been dissatisfied with TSB's communications with its customers. We have had concerns that TSB was not being open and transparent about the issues experienced"
One TSB customer told to BBC Radio 4's that he watched thousands of pounds in wedding savings being stolen from his internet account. As well as he waited on hold for the bank's fraud department.
Alford from Weymouth said it took more than four hours to get through TSB’s transaction. By which most of the money had stolen from internet bank account. TSB also said it had put for the purposes of "additional resources" to support customers.
Lorna Connolly formerly told to the BBC news that how she was unable to access her account days before her wedding day. She had to "ring to grovel" with suppliers for the wedding TSB rang the day before her wedding to offer her compensation of £100.
When TSB split out from the Lloyds Banking Group since the bank was continued to use its computer system while a new one was developed. And, it was ready then TSB moved customers' data from the Lloyds platform to its own.
This was a long-planned disruption to the service of the bank. The bank informed customers about the change and they were unable to use online banking or payment systems on that weekend.
Many customers said they were unaware of the changes and that’s why customers experienced difficulties long after the deadline.
Fraud has become a problem with confused customers and that was tricked into allowing access to their accounts. The FCA said that TSB had failed to refund their money quickly.
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The FCA has got its own team that is embedded with TSB since the crisis unfolded with 14 supervisors from the regulator and the Prudential Regulation Authority deployed.
Nicky Morgan who chairs the Treasury Committee of the bank and said she was "deeply concerned by TSB's poor communications about the scale and nature of the problems. It has faced by its response to customer fraud and by the quality and accuracy of the oral and written evidence provided by Mr. Pester to the committee".
The committee will question to senior members of the FCA and TSB bosses on Wednesday afternoon. For this instance reason, a TSB spokesperson said about this problem that no customers will losses their money