Branches of NatWest have opened across the country on Sunday for the first time as the bank attempts to clears a massive backlog of payments caused by a computer glitch.
Stephen Hester, chief executive of NatWest owner RBS, yesterday issued a public apology for the technical hitch and conceded the bank had let down its customers.
Twelve hundred branches opened their doors at 9am and will stay open until noon, amid continued anger following days of disruption, the Press Association reported.
The chaos, caused by a problem with computer software, left many of its clients unable to pay bills or access their money.
Mr Hester attempted to reassure customers following mounting fears that thousands of people could be hit with penalty charges if their regular payments - including mortgages - were affected.
"I am very sorry for the difficulties people are experiencing," he said.
"Our customers rely on us day in and day out to get things right, and on this occasion we have let them down. This should not have happened.
"Right now my top priority, and the priority of the entire RBS Group, is to fix these problems and put things right for our customers.
"This is taking time, but I want to reassure people that we are working around the clock to resolve these problems as quickly as we are able."
NatWest has more than 7.5 million personal banking customers but it is unclear how many have been affected. The bank has urged customers to keep records in case of losses.
The issues extend to users of Royal Bank of Scotland (RBS) and Ulster Bank, which like NatWest are part of RBS Group.
Ulster Bank said about 100,000 of its customers experienced difficulties.
While the original technical glitch has been fixed, staff are now working through the build-up of transactions which have not been processed.
Double the usual number of employees are manning call centres to tackle the problems, Mr Hester said.
"Our staff have already helped thousands of customers to access cash and we will continue to provide this service on a 24-hour basis while we work to resolve the problems," he added.
"I also want to reassure customers that no one will be left permanently out of pocket as a result of this, and again, they should contact us directly about this."
In a message to staff, he said he was proud of the work his employees were doing but that they were "not out of the woods yet and there is more hard work ahead".
The initial problem reportedly arose following an attempt to install a software update on RBS's payment processing system, which was then corrupted.
The fault meant payments went awry, wages appeared to go missing and holiday and home purchases were interrupted.
When account balances were not updated properly overnight, credit and debit payments failed to show up as quickly as they should.
The upheaval sparked fury among customers who turned to Twitter to vent their anger.
One wrote: "Has anyone actually had anything clear yet? Within the last our (sic) I have had ALL my bills go out ... but oohing (sic) go in! I'm now going to be charged right?? I was told on the phone this morning by one person I would have my money by 8am and then by some one else I would have it by 12pm...whatever next! Now I'm very overdrawn...and still have no available funds!"
Another, Amanda, wrote: "I just feel so sorry for the staff in the call centres. I spoke to a lady this morning who was clearly close to tears.
"It's difficult to remember, but it's not their fault. I did try not to shout at her but it was very hard, particularly after 35 minutes on hold with the silly announcement saying 'check the website' every few seconds.
"The frustration is now getting to everyone.
"How a situation like this can arise is unbelievable. I've had to explain why my rent is late, and they were not at all sympathetic!"