Australian extortionists used a code made famous in the best selling novel The Da Vinci Code to try to blackmail a global construction company, according to a newspaper report.

According to Sydney's The Daily Telegraph, the blackmailer threatened to kill crane operators working for Multiplex unless he received $50 million Australian dollars.

The 400-year-old Vigenere Code was being used by the extortionist in his threats, the paper reported.

The man was communicating with the company via newspaper advertisements and made Multiplex officials use the code to respond.

It was not broken until 1860 and the paper cracked a version of the code on Thursday.

The Daily Telegraph deciphered the message which appeared in The Weekend Australian on Feb. 19.

In its encrypted form it read: "SVGUCSK BFPTAT NSKWEUM DS MZ YXTQA – LV@JXPLBGZCJ.VTS"

It could only be cracked using a key word to translate the cryptic string of letters.

The key, according to the paper, was: "DESTROYMULTIPLEXDESTROYMULTIPLEXDESTROYMULT"

When translated, the message is a bid by Multiplex to have the blackmailer contact them.

It reads: "PROBLEM PLEASE CONTACT ME ON EMAIL AR@MULTIPLEX.BIZ."

Multiplex confirmed the extortion demand in a statement earlier this week, without going into any specific details.

The paper also revealed that the extortionist wrote to the Construction, Forestry, Mining and Energy Union urging them to warn their members of the impending danger, in an apparent effort to shut down construction sites.

The note said something along the following lines: "Multiplex are bastards. They've been ripping off sub-contractors too long. They haven't paid compensation.

"Therefore you better go and tell the workers – because one of them is going to get killed."

More than 1000 workers walked off the job in Victoria and Perth. No workers are reportedly harmed, including those at London's landmark Wembley Stadium, since the blackmail deadline passed.

Company officials said they were working closely with police and unions to take precautionary steps to protect staff.

"Employees have been provided with the opportunity to determine whether or not they wish to continue working," the statement from Multiplex said, noting that the threat did not specify any particular construction site.

The Daily Telegraph reported speculation that the blackmailer was an outraged sub-contractor.