Natwest said that an "update was imminent" but that there was still problems with its system that has prevented customers from getting up-to-date balance and from making payments.
Technical issues with its computers has now meant that payments in or out of accounts had not been made since Wednesday evening.
In a statement it said: "We can assure our customers that this problem is strictly of a technical nature and will be fixed as soon as possible. We can also confirm that no customers will be permanently out of pocket as a result of this."
The glitch, which also affected online banking services, meant that workers were left without their wages.
People also faced fines for late payment of bills because the computer meltdown left them with insufficient funds to honour direct debit arrangements or direct debits payments were not made.
Last night consumer groups called for customers to be compensated.
NatWest kept 1,000 branches open until 7pm yesterday and opened at 8am this morning to deal with customers' complaints. The problem also affected over 100,000 customers at Northern Ireland's Ulster Bank, which is also owned by RBS Group. In addition 223 RBS branches opened early today, the bank said.
The banking group apologised for the "unacceptable inconvenience" it had caused to customers and said it was investigating the causes of the problems.
Sarah Brooks, director of financial services at Consumer Focus, said: "We hope that the problem will be resolved quickly and doesn't happen again. NatWest and Royal Bank of Scotland should also look at providing appropriate compensation to any customer who loses out because of this failure."
Problems with the group's computers meant that people who were relying on money being paid into their accounts, such as wages, were unable to access this cash.
The move left many customers helpless and angry.
Peter Hurst, a NatWest customer who could not access his money, told The Daily Telegraph: "It's all a bit Greek. What effect this will have on the economy, God only knows. You expect this in the Third World but not in London, the so-called business capital of the world."
Meanwhile a customer called Kora-Lee Holmes told Twitter: "Missed my flight home from Greece because NatWest's server problems mean I can't check out of my hotel. New flights (cost) £200."
Megan Batterbee, another customer, said: "Don't worry, it's not like I need to buy food anyway. I've got four pounds to last me as long as this stupid glitch does."
A saver called JustABC told the social networking site: "My balance is reading £0.00 available, so what have you done with my pay? Now your online service isn't working. Unacceptable."
The technical fault did not affect people who had pre-existing sums of money in their accounts yesterday. The banks' ATM (Taiwan OTC: 3423.TWO - news) machines are still working and branches remain open.
The bank took to Twitter yesterday to apologise to its customers for the problem.
"We recognise this has caused significant inconvenience for our customers and has impacted many of our services. This is an unacceptable inconvenience for our customers for which we apologise," the bank Tweeted.