Natwest has admitted via Twitter that some of its customer' account payments had gone missing. Around 600,000 transactions are thought to have been affected.
"Some Customer payments are missing this morning we are investigating this issue as a matter of urgency," it said. The issue affected not only NatWest customers but also those of other banks in the RBS group; RBS, Coutts and Ulster Bank.
RBS confirmed in a statement that it was "aware of an issue with 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," it added.
"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. We will ensure no customers are left out of pocket as a result of this issue."
The bank said that the underlying fault has now been "fixed" but has not said when the missing funds would reappear in customers' accounts.
The news comes after problems affecting customers of the Nationwide Building Society, leaving them unable to access accounts via online banking. In some cases, entire accounts had disappeared.
One Twitter user said at the time that their Flex account "had disappeared from my banking app I can see you are having issues. Will it be sorted soon?"
Another said: "What are the odds that your bank account disappears from online banking on the same day that you just get paid! #payday #Nationwide #unlucky."
While the main problem appears to have happened on Monday morning at around 6.30am, other customers have complained of not being able to access funds in their account since Friday. However, the problem did not affect cash machines or paying by card.
During the outage a statement on its website said "We're sorry to have to tell you that your online Bank is unavailable. We are making every effort to restore service as quickly as possible. We apologise for any inconvenience this will cause you."
The problem comes just after the building society said on its Twitter account that planned maintenance work on its online banking services had been "successfully completed".
In a statement, a spokesman for the building society said that the issue had been "resolved" and apologised "for any inconvenience caused". But Nationwide would not say what caused the problem or how it was rectified.
Colin Mitchell, UK Financial Services and Insurance director at Delphix, said that banks are struggling to balance continual improvements to service with the kind of stability we expect from the pillars of commerce.
"Nationwide is no stranger to glitches but it is by no means alone. This year we've already seen system failures at Tesco Bank, Natwest and Sainsbury's Bank," he said.
Mitchell added that customers are becoming more and more vocal about the frustrations they experience when banking services go down.
"The financial services industry needs to ask itself the question: how are these defects creeping into production? Are financial service providers not testing thoroughly, acting in haste or is their infrastructure simply unable to cope?"
Mitchell said that banks will have to find a way to move faster but not risk their critical reputational capital. To make sure that IT systems are resilient and reliable, "there needs to be more focus on moving towards risk-free testing".
In December 2013, RBS blamed "decades of underinvestment" for failures of its IT systems that left customers unable to access funds or pay for goods. Last year, a glitch hit RBS' mobile app customers near pay day.
