NatWest parent Royal Bank of Scotland admitted around 600,000 banking payments that have failed to go through may not apply to accounts until the weekend.
Taxpayer-backed RBS said the issue with its "overnight process" has been identified and fixed, but admitted affected customers of NatWest, Royal Bank of Scotland, Ulster Bank and private bank Coutts may have to wait until Saturday for the payments to show up in their accounts.
A spokesman 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."
The issue has impacted credits and direct debits and is thought to also include tax credits and disability living allowance payments.
The group was not able to confirm how many customers had been affected, but said the problem did not apply to all accounts.