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A pc failure at TSB that brought on as much as 1.9 million individuals to lose entry to on-line banking providers is being investigated by the monetary regulator.
The introduction of a new IT system in April left prospects struggling to make transactions and see their balances.
The Financial Conduct Authority (FCA) stated the bank's boss, Paul Pester, had given an "optimistic view" of providers.
He is because of seem earlier than MPs for a second time on the disaster later.
FCA chief government Andrew Bailey stated: "We do not normally make this information [the investigation] public, but, given the level of public interest, I want to be clear that we will be conducting this work."
He also stated that the bank's poor communication could have hit prospects' belief in banking.
Some TSB prospects are nonetheless dealing with issues with providers after its 5 million prospects had been moved to a system run by its Spanish proprietor Sabadell six weeks in the past.
In the midst of the disaster, Mr Pester informed MPs on the Treasury Committee that almost all TSB prospects might perform transactions as regular.
In a letter to the committee, the FCA's Mr Bailey acknowledged that view was optimistic and too optimistic.
"Greater caution would have made sense," he wrote, including that Mr Pester ought to have shared extra particulars in regards to the issues with MPs on the committee.
The prospects themselves had been informed by the bank that the overwhelming majority of them might use their on-line accounts, when solely half might achieve this.
Mr Bailey wrote: "The FCA has been dissatisfied with TSB's communications with its customers and we have had concerns that TSB was not being open and transparent about the issues experienced."
One TSB customer informed BBC Radio four's You and Yours final month that he watched hundreds of kilos in marriage ceremony financial savings being stolen from his web account as he waited on maintain for the bank's fraud division.
Ben Alford, from Weymouth in Dorset, stated it took greater than 4 hours to get by to TSB, by which era many of the cash had gone. TSB stated it had put in "additional resources" to assist prospects.
Lorna Connolly, previously Lorna McHale, informed the BBC how she was unable to entry her account days earlier than her marriage ceremony day.
The 26-year-old stated she needed to "ring to grovel" with suppliers for the marriage. TSB rang the day earlier than her marriage ceremony to supply her compensation of £100.
When TSB break up from the Lloyds Banking Group, it continued to make use of its pc system whereas a new one was developed.
When it was prepared, TSB moved prospects' information from the Lloyds platform to its personal.
This was a long-planned disruption to the service. The bank stated it knowledgeable prospects of the change, and that it could result in them being unable to make use of on-line banking or cost programs that weekend.
That led to 2 issues. First, many shoppers stated they had been unaware of the modifications and so had been caught out. Second, prospects skilled difficulties lengthy after the deadline that TSB had promised issues could be fastened.
Many providers have improved, however the state of affairs stays unstable, with 40% of these attempting to name the bank unable to talk to somebody, whereas ready instances have run to greater than 30 minutes.
Fraud has grow to be a drawback, with confused prospects being tricked into permitting entry to their accounts. Yet the FCA acknowledged that TSB had did not refund their cash shortly sufficient.
The FCA has had its personal crew embedded with TSB because the disaster unfolded, with as many as 14 supervisors from the regulator and the Prudential Regulation Authority deployed.
Nicky Morgan, who chairs the Treasury Committee, stated 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 query senior members of the FCA and TSB bosses on Wednesday afternoon.
A TSB spokesperson stated it was "doing whatever it takes to put things right for our customers and ensuring that no customer will be left out of pocket as a result of the recent IT issues".