National Irish Bank (NIB) was forced to recover cash from almost 5,000 credit card customers after an embarrassing mix-up that led to the bank undercharging consumers on currency translations.

The Irish Times learned yesterday that between April 2nd and 6th, the bank mistakenly converted sterling to euro on NIB Mastercard transactions before charging them to the individual accounts. For example, transactions of €120 would have been debited to the accounts as €80.22. The bank explained that this resulted in the customers being undercharged.

It added that a total of 4,956 customers were affected by the mix- up.

However, a spokeswoman said that the bank was last night unable to say how much money was involved.

The bank was not able to say what caused the mistake. The spokeswoman said that it was currently checking all its systems.

A bank statement last night said: "We became aware of this error on April 6th 2004. As a result, we have written to the customers affected explaining the situation and rectified their credit card accounts to reflect the correct amounts".

Its spokeswoman said that all the cash had been reclaimed by April 16th.

A small number of customers who sought refunds on goods purchased with their cards were overcharged as a result of the mistake, as the euro-sterling transaction was reversed.

The bank could not say how many were affected in this way, but the spokeswoman pointed out that typically in transactions of this nature, individual amounts tend to be small.

She added that anyone who had been overcharged would have had their cash returned by April 16th. "Everything was corrected by that time," she said.