Bank of Ireland is to refund tens of thousands of its mortgage customers after it discovered that it had overcharged them.

Around 35,000 accounts were impacted by the error that arose when repayments were miscalculated by the lender.

"This meant that they were paying on average 80c extra each month and the average total reimbursement to customers is €16.35," the bank said.

"We are in the process of writing to customers to notify them of this error, refund them the amount they overpaid to their mortgage, and to apologise for any confusion caused."

The blunder looks set to cost the bank around more than €572,000.

However, around a third of the 35,000 now face an increase in their repayments over the remaining term of their loans.

On average they will have to pay around €2.09 extra on top of what they are currently repaying each month.

This is because they had availed of a flexible mortgage option at some earlier point in their mortgage.

As a result, their future repayments now have to be recalibrated to take account of the bank's error, in order to ensure they still clear the remaining balance by the time their mortgage matures.