Qantas says its chief executive, Alan Joyce, has received a threatening letter related to its current industrial dispute.
The letter comes amid a row between the Australian airline and unions on a restructuring and outsourcing plan that could lead to job cuts.
But officials from two unions have raised doubts about the authenticity of the letter, saying that it was not clear who sent it.
Police are investigating the matter.
According to reports in the Australian media, the letter went on to read: “The unions will fight you… Qantas is our airline, started and staffed by Australians, not foreign filth like you.”
Irish-born Mr Joyce has been Qantas’ chief executive since November 2008.
Luke Enright of Qantas confirmed to the contents of the letter to the BBC, though he refused to comment further on the matter.
However, the Transport Workers Union (TWU) and Australian Licensed Aircraft Engineers Association (ALAEA) accused the airline to turning the issue into a public relations exercise.
“We are unsure whether it came from an angry employee, or it may have been fabricated by the Qantas management to gain sympathy from the public,” Steve Purvinas, federal secretary of ALAEA, told the BBC.
TWU’s national secretary, Tony Sheldon, said: “This is an unsubstantiated piece of correspondence, that was either created by Qantas or sent by any of its 35,000 employees or people outside the company.”
They said the airline had been losing public support because of its plans to restructure its business and relocate jobs outside Australia and as a result, it was trying to garner public sympathy using such tactics.
“The question here is, did they go to the police first or the media,” TWU’s Mr Sheldon said. “They released the letter to the media even before their staff knew about it.”
The airline and the union members have been involved in a dispute that has seen Qantas’ services being disrupted.
Last month, Qantas cancelled 28 flights, while another 27 were delayed after ground staff stopped work for four hours at all major Australian airports.
The union members have been striking against the planned restructuring that will see the airline’s operations expand in Asia.
Qantas has also announced plans to launch two new airlines, including a budget carrier based out of Japan. At the same time, Singapore and Malaysia are being talked about as potential hubs for the other venture.
There have also been concerns that the outsourcing of certain jobs could result in as many as 1,000 job cuts in Australia.