Anglo Irish Bank has inadvertently released information regarding 500 UK business customers.
The bank's Belfast office emailed a North of Ireland client a spreadsheet containing details of customers and transactions worth €12.5bn.
The information includes accounts details and transactions which allow clients swap one type of loan for another. There is nothing unusual about the transactions, which are carried out by most banks.
The bulk of the clients are British based property development companies.
The bank was unaware of the data protection breach until contacted by RTÉ News late last week.
In a statement last evening the bank said the breach was down to human error and that it is taking the matter very seriously.
It said it has informed Britain's Financial Services Authority, the Information Commissioners Office in the UK and Ireland's Financial Regulator. It is now beginning to contact the 504 clients involved.
The bank says the risk from the breach is minimal and clients' identities cannot be replicated.
The bank says it is investigating and taking steps to make sure it does not happen again.
