Crane operators are continuing to work despite the threats

Multiplex has received a letter warning an operator would be shot by snipers unless it hands over 50 million Australian dollars (�20m) by 18 April.

The Australian firm said it is working closely with the police, unions and employees to resolve the situation.

The first threat was received via a letter to its Sydney office last month.

'Act of terrorism'

The Construction, Forestry, Mining and Energy Union, which includes crane operators, has condemned the threats as an act of terrorism and said it was working to investigate the source of the letters with New South Wales police in Australia.

"We will not be bullied by criminals and thugs who have caused such fear and uncertainty in so many families through these death threats," union President Peter McClelland said.

He said union members voted unanimously to continue working at Multiplex sites despite the threats which he said police considered to be quite low.

A New South Wales police spokeswoman said an investigation into the threats is continuing.

Meanwhile the Met said it is continuing to work closely with the Australian police.

Multiplex also built Sydney's Olympic stadium and has operations around Australia, New Zealand, and the United Arab Emirates.