NatWest's online glitches have forced the banks to extend their opening times again, as furious customers vent their anger online.
Parent company RBS, which is 80% state-owned, after being bailed out by taxpayers in 2008 now potentially faces a huge overtime and compensation bill running into hundreds of millions of pounds, according to one analyst.
Affected customers - including those who bank with RBS and Ulster Bank - have taken to forums and Twitter to vent their frustration, threatening to close their accounts once access to their cash is restored.
Some 1,200 main NatWest and RBS branches will be open from 8am to 6pm for the rest of the week after opening yesterday, on a Sunday for the first time, and late last week.
Gary Greenwood, analyst at Shore Capital, said the saga could cost RBS hundreds of millions of pounds.
He said: "The extra cost of branch openings and fee waivers is likely to cost the company tens of millions of pounds, although compensation costs for customers could push the total cost into the low hundreds of millions of pounds."
Customers will not automatically be entitled to extra compensation for the disruption, according to consumer champion Which? but the Financial Ombudsman Service (FOS) has advised them to keep a record of how they have been affected in case they need to complain later on.
However, the bank has promised to refund any charges customers incurred through not being able to pay bills.
RBS CEO Stephen Hester said the bank had let customers down
RBS opened around 1,200 branches yesterday for the first time on a Sunday, while chief executive Stephen Hester issued a public apology for the debacle.
Some customers are unable to view an up-to-date balance, payments such as direct debits for bills have not been made and some wages have not been received.
NatWest has more than 7.5 million personal banking customers but it remains unclear how many have been affected.
More than 1,000 NatWest branches opened their doors from 8am to 7pm today as staff sought to resolve the problems.
But customers were still fuming.
One customer from Glasgow wrote on Twitter: "I have had my bank charges of £12.95 taken this morning but my salary payment which should have gone in today hasn't appeared. Goodbye Nat West, from a customer of 25 years' standing."
Another from Hedgerley in Buckinghamshire said: "I thought the problem had been sorted and you were working on the backlog - how then have my wages not gone into my account today but the direct debit have come out?"
Others wrote on the NatWest website
Susan Allen, director of customer services at RBS Group, insisted that progress was being made and expressed cautious optimism that RBS and NatWest customer account balances would be largely "back to normal" at the start of the working week.
"The knock-on effects of this technical failure mean there will be bumps in the road," she said.
"We will do everything we can to minimise further disruption to our customers."
The computer software problem started on Tuesday night and reportedly arose following an attempt to install a software update on RBS's payment processing system, which was then corrupted.
The group has said this issue has now been fixed.
It has promised that any overdraft fees or charges on current accounts incurred by customers will be automatically waived and has said it will work directly with credit agencies to ensure no-one's credit score is affected.
It will also reimburse customers who had to pay to ring an 0845 helpline number for any cost incurred when doing so.
Stephen Hester, chief executive of NatWest owner RBS, conceded the bank had let down its customers.
His attempt to reassure clients followed 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."