NatWest says it hopes to have resumed a normal service on Monday, following a weekend of chaos for many customers.
Millions of people found their accounts were reading zero due to unforseen "technical glitches", although many were able to remove cash from branches which were opened specially over the weekend to try and clear the backlog of transactions.
The repercussions have extended beyond the organisation's own clients, with one couple who are not NatWest customers left unable to move into their new home.
Others have seen wages and transfers go missing, leaving them with fines or late-payment penalties, whilst others have been unable to go on holiday because they cannot withdraw or transfer funds.
Account balances were not updated properly over the weekend, meaning credit and debit payments were not showing up as quickly as they should.
The issues extend to Royal Bank of Scotland and Ulster Bank users. All three banks are part of RBS Group, which says it has fixed the initial technical problems, but now has a huge backlog to clear.
The company has pledged no one will be left out of pocket. Anyone who incurs penalties because of the problems has been urged to contact their branch.
Susan Allen, Director of Customer Services, RBS Group, said: "I can confirm that we are making progress to clear the backlog of payments and I'm cautiously optimistic that RBS and NatWest customer account balances will be largely back to normal from tomorrow.
"The knock on effects of this technical failure mean there will bumps in the road. We will do everything we can to minimise further disruption to our customers. And we continue to rely on and appreciate their patience as we work through possible disruptions."
She added: "I can confirm today that we have started the process of putting things right for our customers.
We will automatically waive any overdraft fees or charges on current accounts. This will be processed over the next few days.
"We will work directly with credit agencies to ensure no one has their credit score affected.
For all other issues, customers have our commitment that they will not be out of pocket from this issue. We will publish further details on how we will ensure this later in the week."
It is not known how many of NatWest's 7.5 million personal banking customers have been affected.