NatWest customers were struggling to transfer money on Friday, with some saying cash appeared to have "disappeared into thin air".
RBS/NatWest said it was aware of an issue which meant some customers were having problems with its mobile banking app and with online banking running slowly.
In some cases, customers are having issues with transfers being delayed, it said.
The glitch comes on what is payday for many people, being the last Friday of the month, and a day when many are checking their finances as they make preparations for the Bank Holiday.
One Twitter user wrote: "When you transfer money from a #Natwest account to another and it just disappears into thin air."
Another described it as an "absolute disaster".
An RBS spokesman said the issues started this morning. He did not confirm what had caused the problems, but said the bank was working hard to resolve them.
He said: "We are aware that some customers are currently experiencing issues with our mobile app and online banking running slowly.
"Some customers are also seeing issues completing transfers on accounts. We are working hard to resolve this and apologise for the inconvenience caused.
"Our NatWest and Royal Bank apps and online banking are operating, but some customers are experiencing slowness trying to access them.
"Telephony and branch banking are both available. We are seeing that some customers are having issues with their transfers being delayed."