Around 1,000 NatWest branches are to open at 08:00BST to help customers who are still unable to access their accounts because of technical problems.
NatWest said it had failed overnight to solve issues that meant some customers could not access online accounts.
"Unfortunately we are once again experiencing technical issues with our systems," NatWest said on Friday.
As well as some NatWest customers, others with RBS and Ulster Bank accounts have also been affected.
NatWest and Ulster Bank are part of the RBS group.
RBS apologised and said customers would not be left out of pocket.
In a statement on the NatWest website on Friday morning the bank said: "Unfortunately we are once again experiencing technical issues with our systems and account balances have not updated properly overnight.
"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."
Some branches had stayed open late on Thursday to help customers on their way home and staff continued to work through the night.
One small business owner told the BBC that the electronic system to pay staff was not working.
He said that, while the bank's business service centre had put a note on his account to allow him access to emergency cash, there were no communications within the bank as staff in his branch had no record of it and the note was having to be faxed over.
Many customers took to Twitter reporting that they had not been paid and expressing fears that the problem would not be fixed before the weekend.
NatWest has 7.5 million personal banking customers.
The bank did not say how many people had been affected across the group, but Ulster Bank, which along with NatWest is also part of the RBS group, said 100,000 of its customers had been affected by "a major technical issue".
"We are working hard to minimise the impact and will ensure that those who are affected do not suffer any financial loss," Ulster Bank said in a statement.
Bob Spearman from Petworth told the BBC his weekly tax credit payment had not been paid into his NatWest account.
"We, like many other low-income families live from week to week and the Child Tax Credit weekly payment is a lifeline on which we rely," he said.
A spokeswoman for UK Payments Council, which oversees payments strategy, said that the problem did not appear to have affected any other banks.
However, people expecting payments from individuals or businesses which banked with NatWest could encounter problems.
Last week, Royal Bank of Scotland and NatWest launched a mobile banking app to enable people to withdraw money from cash machines using their smartphone.
This marked the latest development in a long-predicted move towards the smartphone becoming a digital wallet.