State-owned AIB has been forced to refund thousands of business customers after it overcharged them.
The bank has sent letters telling almost 7,000 customers that they are due a refund amount, including interest and fees that are being repaid to them.
The average refund amount is €543, the bank said in a statement.
Business customers thought they had overdraft facilities in place. But the bank did not recognise this, and the customers ended up being charged surcharge interest and penalty fees for having an "unauthorised" overdraft.
It is understood the bank spotted the error and informed the Central Bank about it.
In a statement, AIB said: "Following a recent review by AIB of its overdraft product, it was highlighted that a number of issues may have resulted in an unintended error in the amount of surcharge interest and/or fees being charged to the accounts."
The bank said it was refunding 6,885 customers.
"AIB sincerely apologises for this error and any inconvenience caused as a result. The appropriate changes to procedures have been implemented," the bank said.
The Central Bank confirmed it was aware of the issue.
It said the Consumer Protection Code required errors to be resolved speedily and for customers to be notified and refunded appropriately.
The Central Bank did not say if it would take any further action. It advised those who suspect they were overcharged customers to contact AIB.
This is just the latest overcharging issue to affect the bank.
Last April, this newspaper reported that a major mortgage blunder by AIB has left thousands of families out of pocket, with some ending up overdrawn.
The error cost families up to €375 each and the bank did not immediately rectify its mistake.