Seven people have been arrested on suspicion of being illegal workers after a raid on the Queensferry Crossing construction project.
The raid, conducted by the Home Office Immigration, Compliance and Enforcement team, took place on Monday.
The individuals - who are understood to be from the Indian sub-continent - were not employed directly by Forth Crossing Bridge Constructors (FCBC) but through a second tier labour-only subcontractor.
However, union UCATT claims the incident is just one example of wider workplace abuses on the site.
Steve Dillon, Regional Secretary of UCATT Scotland, said: “This flagship project is now operating like the Wild West. Without a union convenor the site has no sheriff and this is inevitably going to increase the exploitation and mistreatment of workers.”
Mr Dillon added: “To restore confidence on the project the Scottish Parliament must tell the contractors to give unions full access to the site and to re-appoint a convenor to deal with problems as soon as they occur.”
UCATT said it also believed southern European workers employed by sub-contractor Sosa on the project are being paid below industry agreed rates.
FCBC, the project contractor, has co-operated fully with the investigation.
Keith Brown, cabinet secretary for the economy, said: “I have written to the Home Office Minister Robert Goodwill requesting an urgent discussion and to seek reassurance over the measures in place to address the issue of the use of illegal foreign workers.
“It’s important that the construction industry can responsibly provide the correct resource to support the delivery of our pipeline of infrastructure projects.
“Across our projects, contractors have assured us that they carry out all business and operations in such a manner as to fully comply with and meet all legislative requirements, including all relevant employment laws.
“As such, it is standard policy to carry out checks to ensure that all direct employees and staff have the necessary and valid credentials as is required for them to be legitimately employed prior to their appointment.
“We have also received assurance that it is also a requirement that second tier subcontractors also meet these obligations.
He added: “As we would expect, the project contractor Forth Crossing Bridge Constructors (FCBC) is fully co-operating with the Home Office investigation, we await the outcome of this investigation and it would be inappropriate to comment further.”
The seven men are expected to appear before Dunfermline Sheriff Court.