NatWest customers face further upheaval after technical hitches left angry clients unable to pay bills or access their money.
The bank has taken the unusual step of opening hundreds of branches over the weekend and said it was working "round the clock" to clear a backlog of chaos.
More than 900 branches will be staffed until 6pm today and until midday tomorrow.
It remains unclear exactly when the issues will be completely resolved.
In an email sent to customers, Natwest apologised and attempted to explain the problems:
"We apologise to all of our customers who have been affected by our current technical issues," it read.
"These resulted in money credited to customers accounts overnight not appearing in their balances as it should. This problem is strictly of a technical nature, we have fixed the initial problem and our priority has been to work round the clock to sort the backlog as soon as possible. "
The repercussions have extended beyond the banks' own clients, with one couple who are not NatWest customers left unable to move into their new home.
Hundreds of customers have vented their frustration over the problems, with some finding payments had gone awry and others observing disappearing wages, or holidays and home purchases disrupted.
The group has apologised and moved to quash fears over withdrawals, assuring customers their money is "in the system" and staff will be on hand in branches to help them access their cash.
It 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.
NatWest has more than 7.5 million personal banking customers, but the group said it could not tell how many have been affected as it was not possible to know when they were expecting payments into their accounts.