An investigation by construction union UCATT claimed that joiners working for Portuguese sub-contractor Sosia Ltd were being paid £7.67 an hour and labourers were receiving £6.32 per hour, well below the agreed minimum rates of £11.61 and £8.73 respectively.

Labour MSP Neil Findlay then called for an immediate pay audit to be launched across the entire project at First Minister’s Questions on Thursday last week.

Mr Brown said Acas will work with the lead contractors Forth Crossing Bridge Constructors (FCBC), Transport Scotland and UCATT top investigate the allegations.

He said: “I advised UCATT that I had ensured that their allegations regarding low pay had been fully investigated, with no evidence being uncovered to substantiate the allegations.

“In response, UCATT have advised that they remained of the view that such practices had been prevalent.

“I have therefore offered to ask Acas to undertake further work in conjunction with FCBC, Transport Scotland and UCATT to examine this and related issues. UCATT have agreed with this approach.

“It remains the responsibility of our contractor FCBC to manage matters relating to its workforce and sub-contractors. The Scottish Government will continue to work together with all parties to ensure that all obligations over workers’ rights, health and safety and pay are being met on the Forth Replacement Crossing project.”

Meanwhile, Forth Replacement Crossing project director, David Climie, said FCBC are “thoroughly investigating” the claims but has so far found no evidence of wrongdoing.

He told Holyrood’s rural and connectivity committee that payslips belonging to some of Sosia Ltd’s 29 staff working on the site have been checked to see what they have been paid.

He said: “All those investigations to date show that all the rates of pay are at or above the correct rates and there are no incorrect deductions being made.”

He added: “To date absolutely nothing has been found to back up those allegations regarding low pay.”