TSB's IT fiasco is heading for a fifth day, with angry customers still locked out of their accounts and unable to make payments.
The bank said it would take down its mobile app and online banking "for a few hours" on Tuesday and hoped to restore services by the afternoon.
But it now looks unlikely the issues will be resolved until Wednesday.
Chief executive Paul Pester said he was "deeply sorry" for the chaos and that no one would be left out of pocket.
The bank's data breach on Sunday, which allowed some TSB customers to see other people's account details, has also attracted the attention of the Information Commissioner.
I've just resurfaced after 48 hours with my teams who have been working as hard and fast as they can to get our services back up and running.— Paul Pester (@PaulPester) April 24, 2018
Mr Pester said: "This isn't the level of service that we pride ourselves on providing, and isn't what our customers have come to expect from TSB, and for that I'm deeply sorry."
How are customers affected?
People are beginning to report a wide range of financial problems caused by being unable to access their account online.
Colin Patterson of Manchester told the BBC: "The lack of an official statement, other than generic Twitter replies, is outrageous." He's been trying to pay for his passport renewal for three days and reports that "it keeps being rejected".
"I get paid tomorrow and don't know if I'll be able to pay my rent. It's a disgrace."
Trevor Brittain from Tonbridge said: "I've been with the same branch for 42 years. It was Lloyds but transferred to TSB at the sell-off. I will be switching back to Lloyds as soon as I can get access to my account."
Jessica Callaghan from Newcastle said: "I'm angry because they aren't communicating. They just keep saying, 'sorry, it'll be back up shortly'. I want them to give an official statement about what is happening, and compensation for each customer."
London photographer Paul Clarke is worried that his business will be hit if the situation isn't resolved soon.
"I have no idea who's paid me recently and I can't pay suppliers," he told the BBC. "If we get to the middle of the week and it's still not resolved then my reputation and business could suffer as I work on a quick turnaround. It's a fiasco."
Amanda Verbaan-Dunn, owner of the Wilton Hotel in Blackpool, said: "I'm away from home and can't pay my staff. I'm going to have to go back three days early so I can sort my staff out. It's extremely frustrating and the lack of communication is inexcusable."
Lee MacDonald, who played Zammo in children's TV show Grange Hill in the 1980s, told the BBC he has had an "absolute nightmare" following TSB's IT glitches.
Mr MacDonald, who now runs Mentor Lock and Safe Company in Wallington, Surrey, told BBC Radio 5 live:
"I'm having an absolute nightmare. I've got two accounts with TSB, both business accounts. My business has literally stopped. I don't know what money's coming in, what's coming out.
"When you're running a small business, every job counts and it's just a nightmare."
What has happened?
When TSB split from the Lloyds Banking Group, it continued to rent a banking platform from its former owner while it developed its own "state-of-the-art" platform.
That was ready to go live last weekend and TSB moved its customers' data from the Lloyds' platform to its own. This was a long-planned disruption to the service and the bank said it informed customers of the change and that it would lead to them being unable to use online banking or payment systems at the weekend.
It promised that systems would be back up and running by 6pm on Sunday but many of those who did get access to their online accounts were presented with details of other people's accounts too.
Since that data breach many customers have reported being unable to log on, while those that have been able to log on say there have been problems with passwords or that they have been logged out.
What could happen to TSB?
The Financial Conduct Authority, which has the power to fine banks for system failures, said it was in "regular contact" with TSB.
"We are working with the firm to ensure customers are properly communicated with and are not left out of pocket."
"We will be talking to the firm to understand exactly what went wrong and the steps that they are taking to ensure something like this does not happen again," a spokesperson said.
The Information Commissioner's Office also said it was "making enquiries" about a "potential data breach".
Nicky Morgan MP, chair of the Treasury Committee, weighed into the crisis on Tuesday by writing to TSB boss Paul Pester to find out what has gone wrong, the extent of the failure, and how the bank intends to compensate customers who have suffered a breach of potentially highly-sensitive personal data.
Mrs Morgan said: "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.
"Warm words and platitudes will not suffice. TSB customers deserve to know what has happened, when normal services will resume, and how they can expect to be compensated."