A report has called into question whether UK lender TSB carried out enough testing before the failed IT migration that led to chaos for the bank's customers.
The preliminary report by technology giant IBM said the migration involved "compounded risk" that demanded "extensive engineering, testing and proving".
IBM had been called in during the first week of the meltdown by the bank to help fix the crisis.
Its report today stated that "performance testing did not provide the required evidence of capacity".
"IBM has not seen evidence of the application of a rigorous set of go-live criteria to prove production readiness," it added.
The IBM report goes on to compare TSB's situation with a migration at another financial institution where "multiple trial migrations were conducted, rolled back and then remediated prior to launch".
IBM presented the report to TSB's board on April 29, three days before the bank's chief executive Paul Pester appeared before the UK Treasury Select Committee.
He told the committee that all prior testing "gave us belief that we were ready for the migration".
The report, published by the committee, has been described by Financial Conduct Authority chief executive Andrew Bailey as a "first cut of the issues".
This document includes the note that it was a "preliminary work plan with very early hypotheses, produced after just three days of engagement with TSB" he said.
"These hypotheses were not final nor were they a validated view of what went wrong or of the actions that may or may not subsequently have been taken," he added.
Earlier this month the committee said it had "lost confidence" in Mr Pester and warned that if he continued in his position it could "damage trust" in the bank and the rest of the sector.
Mr Pester had "not been straight" with customers at the bank since its online services were plunged into chaos, it said.
In a letter to bank chairman Richard Meddings, the committee said: "The committee considers that the TSB board should give serious consideration as to whether Dr Pester's position as chief executive of TSB is sustainable."
Up to 1.9 million people using TSB's digital and mobile banking found themselves locked out of their bank accounts following the migration of data on customers from former owner Lloyds' IT system to a new one managed by current owner Sabadell.
"The IBM document contained a preliminary work plan with very early hypotheses based on observations to date, that were produced after only three days of engagement with TSB," a TSB spokeswoman said.
"To present this document as a clear view on what went wrong wouldn't be a fair reflection. Similarly it isn't a fair reflection of what actions may or may not subsequently have been taken," she added.