The IT chief of TSB Bank failed to inform board members about "shortcomings" with the testing of a new technology system days before it crashed - resulting in nearly two million customers losing access to their current accounts.
Sky News has learnt that a long-awaited report by the City law firm Slaughter and May on TSB's IT meltdown last year will say that Carlos Abarca made an "ill-judged" assessment of the lender's readiness to proceed with the migration.
The report is expected to be published on Tuesday.
The subsequent crisis cost TSB's chief executive Paul Pester his job, and the bank £370m in customer compensation and expenses related to the long-running inquiry.
Mr Abarca is understood to have been identified in the Slaughter and May report as having failed to escalate concerns about the testing of the new IT framework - which was created to transfer millions of customers from a system created by Lloyds Banking Group to one designed by TSB's Spanish owner, Sabadell.
Sources said this weekend that Mr Abarca's move in March this year from being TSB's chief information officer to chief technology innovation officer had been significant because he was no longer subject to the financial watchdog's Senior Managers' Regime (SMR).
Under the regime introduced by the City regulator in 2016, executives now bear personal accountability for their actions in banks and insurance companies, and have to sign attestations relating to decisions they take.
It was unclear whether Mr Abarca's transition to a role not subject to the SMR was undertaken by TSB in order to shield him from the potential consequences of the IT meltdown.
Last month, the Treasury select committee warned that regulators risked being perceived as ineffective because no bankers had been the subject of successful enforcement action over IT failures in the industry.
Any suggestion by the Slaughter and May probe that board members were kept in the dark about the IT upgrade's prospects of success will raise other questions about the assurance and governance processes followed by directors.
It may also, however, make it harder for regulators to pursue enforcement action against other TSB executives.
The law firm's inquiry is to be followed by a joint investigation by the Financial Conduct Authority and Prudential Regulation Authority into the TSB crisis.
To date, Mr Pester is the only senior figure to have lost his job over the fiasco, although a number of other managers have also collectively had millions of pounds in deferred bonuses cancelled by TSB's remuneration committee.
Sky News revealed earlier this month that the final cost of the Slaughter and May report had hit £25m - equating to roughly £83,000 for each of its nearly-300 pages.
Its publication will come less than a week before a strategy update from Debbie Crosbie, Mr Pester's replacement as chief executive, that will involve substantial job cuts and branch closures.
The failure of TSB's new IT platform sparked hundreds of thousands of customer complaints and led to TSB reporting a pre-tax loss of more than £105m last year.
Mr Pester said a few days into the crisis that the bank was "on its knees".
He subsequently faced criticism from Andrew Bailey, the FCA chief executive, for failing to be "open and transparent" about the IT issues.
Mr Pester has already surrendered one £1.6m instalment of a deferred share-based Sabadell Integration Award, with no TSB executives receiving bonuses for 2018.
Mr Pester and Mr Abarca will not be the only individuals singled out for criticism in the Slaughter and May report.
Bosses are understood to be accused of a failure to ask the right questions about the process - which represented one of the most difficult technology transitions ever attempted.
Earlier this year, The Sunday Times reported that Sabadell's in-house IT services unit had breached its delivery contract with TSB and had provisionally agreed to pay the British high street bank more than £150m in compensation.
The terms of reference for the inquiry, which were published by TSB some time ago, made it clear that Sabadell would not actively contribute to it.
However, sources added on Sunday that the report would say that Sabis, the Spanish group's in-house IT services arm responsible for designing the new system, had taken an obstructive approach to information requests.
In its annual report this year, the lender said "the underlying issues related to some aspects of three key interconnected issues - the initial configuration, the capacity of the infrastructure and also some aspects of coding".
According to TSB's accounts, the IT meltdown has already cost the bank close to £370m in "post-migration charges", including the cost of the investigation.
The majority of these charges related to compensation for customers who were left out of pocket by the IT problems.
TSB received a £450m dowry from Lloyds to take over the business, which it was forced to divest under a state aid deal in the aftermath of its £20bn taxpayer bailout in 2008.
A TSB spokesman said on Sunday that it would not comment on criticism of individuals in the report ahead of publication.