Precautions have been taken following the threat

Reports said snipers would open fire on building sites across the world if Australian company Multiplex failed to hand over �20m.

Multiplex said it had tried to inform all staff after extortion demands were made in a letter to its Sydney office.

The Met is monitoring the situation to "minimise any potential risk".

Multiplex, which also built Sydney's Olympic stadium, said the threat was made last month against company employees but did not specify a particular site.

Workforce safety

The company would not comment on the contents of the letter, or the demand.

A statement read: "Employees have been provided with the opportunity to determine whether or not they wish to continue working.

Multiplex projects Wembley Stadium Retail development in White City, west London Residential development in Knightsbridge, central London Sydney's Olympic Stadium Melbourne's Federation Square Dubai Marina in United Arab Emirates

"Ultimately 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 all UK sites continue to operate on a business as usual basis."

'Assurances'

A Met spokeswoman said: "The Met police is working closely with the Australian Police, the victim and the unions to monitor the situation and minimise any potential risk."

A spokeswoman for manufacturing union Amicus, which has 400 members working on the new stadium in north-west London, said: "We have had assurances from the police and from the company that every security precaution has been put in place."

Multiplex employs about 1,800 people throughout Australia, New Zealand, the UK and the Middle East, and has been involved in some of Australia's biggest construction projects, including Melbourne's Federation Square and the NSW police headquarters in Sydney.

The �757m Wembley Stadium is due to be officially opened for the 2006 FA Cup Final.