Claims by Aer Lingus management that some airline staff are stealing from passengers have been described as "outrageous" by Siptu.
The union, which represents almost 1,000 Aer Lingus staff, described the reports as published in the Sunday Independent, as "blanket character assassination".
The newspaper quotes the airline's chief operating officer Mike Rutter in an internal memo stating that "many millions of euro" is lost by the airline due to missing stock and the losses "remain at levels significantly above the industry norms despite investment in new technologies and inventory management processes".
Mr Rutter went on to blame a “small percentage” of staff for the losses.
In a statement issued on Sunday evening Siptu described the comments as an “outrageous claim”.
The union added: “Such blanket character assassination is unacceptable and the union has called on Aer Lingus management to provide evidence as to whether there is any basis to the claims and to follow appropriate procedures, due process and, if required, report them to the appropriate authorities.”
The union is seeking a meeting with outgoing Aer Lingus chief executive Stephen Kavanagh about the memo.
Siptu also states that other unions within the Aer Lingus group of unions have expressed their concern at the claims as outlined in the memo.
In a letter to the airline recently, Siptu complained about what it saw as excessive surveillance with the installation of CCTV cameras in work areas, and more security patrols.