Royal Bank of Scotland and Natwest account payments have gone "missing" following an overnight technical glitch.
The issue is being investigated as a "matter of urgency", with more than half a million transactions across RBS, NatWest and Ulster Bank said to be affected.
RBS said the glitch itself had been fixed, but warned that it would take time for the transactions to show as normal.
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Customers have complained that payments – including tax credits and disability living allowances – have failed to go into their accounts, with HMRC reportedly experiencing a higher volume of calls.
RBS has apologised to customers and said it was working as quickly as it could to resolve the problem.
"To any customers concerned about the implications of this issue, we advise them to get in touch with our call centres or come into a branch where our staff will be ready to help," said a spokesman. "We will ensure no customers are left out of pocket as a result of this issue."
Just before 8am today, Natwest tweeted that "some customer payments are missing this morning" but said that "funds will show as normal once the system is back to normal".
Sky News points out that the group has been "plagued by embarrassing technical glitches and IT failures" in the last few years.
It was fined £56m by regulators after computer system failures prevented millions of NatWest, RBS and Ulster Bank customers from accessing accounts in both the UK and Ireland in 2012. Then, in 2013, RBS's online service was disrupted by a denial-of-service attack.
RBS's most recent annual report stated: "The vulnerabilities of the Group's IT systems are due to the complexity of the Group's IT infrastructure, attributable in part to overlapping multiple legacy systems resulting from the Group's acquisitions and the consequential gaps in how the IT systems operate, and insufficient investments in IT infrastructure in the past, creating challenges in recovering from system breakdowns."
