Branches of NatWest bank were forced to open early on Friday to offer extra help to angry customers as technical issues plagued the bank's online system for a third day.
Social media sites such as Twitter were swamped with angry messages from customers who have found themselves with insufficient funds, despite being paid.
The problems started on Wednesday but a spokesman said customers would not be permanently out of pocket.
A "large number" of customers affected by the problem, which saw many unable to make transfers, check balances or even access their accounts.
The problem is believed to have been caused by the bank's failure to properly update customers' balances over Wednesday night.
"Unfortunately we are once again experiencing technical issues with our systems and account balances have not updated properly overnight," a statement on the bank's website read on Friday morning.
"This means where money has gone into a customer's account, there may be a delay in it appearing on their balance.
"We can assure our customers that this problem is strictly of a technical nature and we continue to work hard to resolve this."
One Twitter user wrote: "#natwest what are u doing! I'm off on holiday today meant to get paid today so where's my money? helpful banking ay!"
Another said: "Due to complete on flat purchase today. All the money missing in cyberspace. Stressful enough without this, thanks #natwest."
Branches across the UK opened at 8am on Friday morning to help customers while the technical hitch persists.
Almost the same number of banks stayed open until 7pm on Thursday evening in an attempt to help customers.
NatWest had only recently launched a new app for its 7.5 million customers, allowing them to access their online banking on their smartphones and take money from cash points using their phone.