Multiplex boss Noel Henderson said he thought a threat to blackmail the contractor out of million of pounds could be a hoax,

The contractor confirmed that it had been warned that its crane drivers might be shot unless it paid a substantial amount of money to a blackmailer.

Henderson said the threat may be a hoax – although the company was taking the man’s demands “very seriously”.

Henderson said he thought rumours that the Russian mafia were involved were unfounded.

He added that the contractor was liaising closely with the police and had also briefed construction unions over the matter.

Police in Australia have been working with Scotland Yard over the past two weeks as a result of the International Olympic Committee’s visit to Wembley last week.

Henderson said rumours the Russian mafia were involved were unfounded

In a statement, Multiplex said: “Employees have been provided with the opportunity to determine whether or not they wish to continue working.

The statement said: ”The safety of our workforce is paramount and we continue to monitor the situation in consultation with police, our employees and the unions.

“Briefings have taken place on Multiplex sites and the majority of the sites continue to operate on a business-as-usual basis. The threat against the company is being taken seriously and inquiries are continuing.”

It is understood that Multiplex has introduced extra police and security to the site to counter the fears of the crane operators on site. It is also understood that some crane operators have refused to work on site.