At least Sabadell seems pleased with its efforts, claiming last night that the digital platform Proteo4UK - based on Sabadell's Proteo platform - would integrate more than 5.4 million customers and provide greater operational simplicity.
The bank has spent thousands of man-years and millions of pounds on the migration, which has been done in stages.
The bank, which offered its customers access to emergency cash at its branches, said it had fixed the issues to facilitate the recovery of the funds overnight and that the problem was the result of a human error, not connected to a 2012 IT issue.
It has previously blamed high volumes of customers trying to log in to the new systems.
TSB customers are still experiencing issues with their mobile and online accounts after a system upgrade over the weekend led to disaster. Proteo4UKwas rolled out to the bank's staff in November 2017 with a full range of banking services, with the full roll-out slated for first quarter of this year.
Sky News has asked the bank to check whether it had contacted them by email since the login problems began.
There have also been reports that TSB customers could access the accounts of other TSB customers, including their account numbers, sort codes and transaction histories, and possibly having the ability to perform actions on these accounts, including transferring money.
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Some supported TSB's assertion that logging in was now possible though full service had not resumed.
Kevin Rohan says: "It's absolutely ridiculous that a big company like this can get something so very, very wrong".
IT Pro approached TSB for comment, but the bank did not respond at the time of writing. Please. My wages went in on Friday and I need to pay rent and other DD. I tried calling was on the phone 45mins and then got cut off.
It later emerged that the Treasury Committee of MPs, which has previously demanded better standards of banks following similar IT glitches, had written to Mr Pester to seek answers.
The FCA has the power to fine banks for system failures, and the regulator has said it was "aware of the issue" and in touch with TSB.
"It simply isn't good enough to expose customers to IT failures, including delays in paying bills and an inability to access their own money", Morgan said.
"Warm words and platitudes will not suffice".
TSB said 402 customers had access to some data they would not normally be shown on Sunday during a "20-minute window".