RBS and NatWest customers have been unable to check accounts online because of problems caused by maintenance work, the Royal Bank of Scotland has said.
Problems arose after the maintenance work went wrong and meant account balances were not updated overnight.
Some accounts have not been credited when they should have been and there have been problems for some customers making withdrawals from cash machines.
In a statement, the bank apologised for "any inconvenience caused".
The Royal Bank of Scotland hopes to resolve the issue within hours
In its statement, it said: "We are working to restore service as soon as we can and will update as soon as we have more information."