Immigration officers raided the site on Monday and removed the workers, who were from the Indian subcontinent and employed by a labour-only subcontractor rather than the main contracting consortium, Forth Crossing Bridge Constructors (FCBC).
FCBC is said to be co-operating with the Home Office's investigation.
However, construction union Ucatt said that the immigration raid was just one example of wider workplace abuses on the £1.35bn project. The union claims that incidents on the site have already begun to increase since FCBC decided not to replace the Ucatt convenor last month.
Ucatt believes that southern European workers employed by subcontractor Sosa on the project are being paid below industry agreed rates and has submitted enquiries to FCBC.
Ucatt regional secretary Steve Dillon 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. 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.”
Keith Brown, the Scottish government’s economy secretary, 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.”
More than 10,000 people have worked on the construction of the Queensferry Crossing since work began in 2011. The bridge is due to open in May 2017.