The raid, conducted by the Home Office Immigration, Compliance and Enforcement team, took place at the Scottish Government’s flagship project on Monday.

The individuals, who are understood to be from the Indian sub-continent, were not employed directly by project contractor Forth Crossing Bridge Constructors (FCBC) but through a second tier labour-only subcontractor.

Construction union UCATT claims the incident is just one example of wider workplace abuses on the site. The union said it is trying to get answers from FCBC concerning the pay being received by Southern European workers employed by sub-contractor Sosa on the project. It is understood the workers are being paid well below industry agreed rates.

According to the union, workplace abuses on the site have already begun to increase since the consortium decided not to replace the UCATT convenor in October. The UCATT convenor was able to operate as a watchdog and enforcer on a site which, due to its size, nature and management opposition was difficult to police.

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.”

FCBC 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 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 man are expected to appear before Dunfermline Sheriff Court.