Mandate says it is "horrified" that such sensitive financial information could be sent to an individual even in error. "The union is very concerned at such a serious security breach by Bank of Ireland. It is just as well that it was delivered to an honest citizen who didn't use the information to his own ends, " said a Mandate spokesman.

The man told the Sunday Tribune that the second page of the Mandate bank statement was attached to the first page of his own statement.

The union's statement included account names and numbers which could have facilitated theft or fraud.

The individual's name begins with 'M' and he uses the same Bank of Ireland branch in O'Connell Street in Dublin as Mandate.

"This is a highly unusual and irregular occurrence and we would like to apologise to the union, " said a Bank of Ireland spokeswoman, adding that there would be an investigation into how the error occurred.