The extent of the overcharging has yet to be determined, but one industry source suggests the figure may be well over 1m. The bank is currently investigating how many of its customers were overcharged, and by how much.

The financial regulator has been contacted by the bank about the matter and is being kept informed as to how the investigation is progressing.

The overcharging resulted from a systems failure in identifying customers who were entitled to discounts on charges, according to a Permanent TSB statement to the Sunday Tribune.

The bank had advertised and introduced a system whereby customers with more than one account . . . for instance, a current and mortgage account . . . would be entitled to discounts. In the case of thousands of customers, these discounts were never credited to their accounts.

The industry source said the discounts were only credited when the customer specifically asked for them, and not automatically. Failure to notify customers of extra entitlements in this manner has been a feature of banking in the Irish market. The bank's statement said the overcharging was discovered in a recent audit.

"Over the past number of weeks, Permanent TSB has made payments of an average of 38 each to a number of customers as part of a project to reimburse customers who may not have received the reductions on their bank fees to which they were entitled. Further payments will be made to other customers in the weeks ahead, " it said.

The reimbursements being paid include a discretionary sum "for any inconvenience caused to you, " according to letters of apology sent out by the bank to customers.

The bank, the state's largest mortgage provider, said it had taken steps to ensure there wouldn't be a recurrence of the problem.

"In order to clear up such situations, Permanent TSB has initiated a project to identify 'linked' accounts and to establish whether the relevant customer is entitled to receive any discount on charges which might have been applied and to organise a repayment of such charges where appropriate."