RBS has alerted customers that up to 600,000 overnight payments may be 'missing' from bank accounts at NatWest, Coutts and Ulster Bank due to a technical glitch, sparking a string of complaints over "yet another Natwest blunder".
RBS said in a statement that the issue, which "resulted in a delay to payments and Direct Debits being applied to some customer accounts", was now resolved, but it could take days to clear up.
It said: "We have fixed the underlying issue, we apologise for the inconvenience caused and we are working flat out to get these payments updated for our customers no later than Saturday.
"We will ensure no customers are left out of pocket as a result of this issue."
Around 600,000 payments may have failed to enter accounts, the BBC said.
A number of customers replied to the notice to complain about the problem, with some worried about facing charges for late payments of bills.
Some Customer payments are missing this morning – we are investigating this issue as a matter of urgency. 1/2— NatWest Help (@NatWest_Help) June 17, 2015
If you have been affected and need to access funds today, please contact us 03457 888444 or visit your branch 2/2— NatWest Help (@NatWest_Help) June 17, 2015
One user tweeted: "Mines missing hurry up got bills to pay and over draft from you can't pay till you get my money back."
@NatWest_Help mines missing hurry up got bills to pay and over draft from you can't pay till you get my money back— Kirsty (@Ajoite71crystal) June 17, 2015
Another said: "I'm glad losing our money is an urgent matter for you #useless #changingbank #beingfinedforit"
An RBS spokesman said there had been "an issue our overnight process which has resulted in some of our customers not having credits or Direct Debits being applied to their accounts".
"We are working to get this resolved as quickly as possible and apologise to customers for the inconvenience caused... We will ensure no customers are left out of pocket as a result of this issue."
The embarrassing incident follows a spate of disruptions at the bank.
In March some customers' wages did not appear to have arrived into accounts and cash transfers were not going through.
In 2013 it was subject to a denial of service attack that saw online card payments blocked.
NatWest said the number for its customers to call for help with the issue was 03457 888444.