TSB has apologised, again, as customers complain of problems with their mobile and online accounts, including claims some had "access" to other people's bank details.
One customer said he could see other people's accounts totalling more than £20,000, while another reportedly discovered he had been wrongly credited with £13,000 after logging back in.
It came several hours after the bank had warned its account holders that some of its services, including online banking, making payments or transferring money, would not be possible over the weekend because of a system upgrade.
The upgrade window was scheduled between Friday at 4pm and Sunday at 6pm though it was clear systems were still down on Monday morning - to the fury of customers.
TSB said on Monday afternoon there were still "intermittent issues" with its services.
It blamed "large volumes" of customers trying to access their accounts and insisted it was still "working as hard as possible" to restore its systems.
The Financial Conduct Authority said it was aware of the disruption and was in touch with bank officials.
One Twitter user, Tracy Hannah, wrote: @TSB as soon as I can get access to my own money I'm closing my accounts down. Not been able to get money since Friday but you can still access payments, tsb is the worst bank I've ever dealt with. #tsb."
Others used more colourful language to describe their frustration.
TSB was yet to explain evidence of data breaches during the mayhem.
Among those customers contacting TSB's social media included several who said they could see other people's accounts after logging back in.
Craig Malcom tweeted: "@TSB I currently have access to #20k+ of other peoples money.
"I suggest somebody answers the phones as iv been on hold for 45 minuets!
"This is a MASSIVE breach of data protection! If i have access to their account they could have to mine as well!"
Another Twitter user called Bex said: "@TSB so go to my app and have someone else's accounts there!!!! Serious dpa (Data Protection Act) breach! Want to speak to someone now and a half an hour wait!! What would the FCA (Financial Conduct Authority) say about this!!! Might just inform them a bank is giving away other people's account numbers".
Sky News was attempting to contact TSB for more information but was yet to speak to a spokesperson who could offer answers on both the disruption and wider data issues.