RBS Banking Group faces fresh compensation claims from millions of customers who were unable to withdraw cash, pay for goods and services, or carry out telephone or online banking on Wednesday night.
It was the second time in nine months that technical problems had left customers of RBS, NatWest and Ulster Banks unable to access their own money.
The bank - which is 80 per cent state-owned - blamed a “hardware fault” for causing its systems to crash but has promised to compensate customers who experienced problems because of the disruption, which lasted for two hours.
Customers began reporting problems with online banking, phone banking, cash withdrawals and debit card payments at around 10pm on Wednesday.
RBS said all services had been returned to normal by 1am today but some customers said the could still could not access their accounts at 9am.
In June last year a software upgrade at the bank went wrong and left millions of NatWest, RBS and Ulster bank customers without any access to their money, with problems lasting for more than a week in extreme cases.
The compensation bill then topped £175m but the hit to the bank’s reputation this time could prove more costly. Today social networks were flooded with complaints from fed-up customers who vowed to switch accounts to rivals.
Phil, who tweets as @wheatear9 said: "Why oh why have I stayed with #naffwest I should know better and will depart." Andrew Bissett, meanwhile, tweeted: "Disgraceful service. Am moving my banking to Santander! You cannot be trusted with our money!!!"
Anthony Gray said: "No money on my Oyster card last night. No way of getting money out. Had to walk home. Will now spend today changing banks."
Staff at the bank's Edinburgh headquarters said it would deal with compensation claims on a case-by-case basis.
One customer, Darren Reuben, has already been compensated.
He told the BBC that he had to get someone else to pay for his dinner yesterday evening. He was initially offered £30 compensation, but when he declined, he was offered £70.
"It was paid into my account straight away," he told the BBC. "While it was embarrassing, they did me a good turn."
Which? executive director Richard Lloyd said: “RBS must quickly explain how affected customers will be compensated and what measures they are taking to ensure this doesn't happen again.”