An investigation by construction union UCATT has claimed joiners 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 levels.

In accordance with the Construction Joint Industry Council (CIJC) agreement, the minimum rates for workers on the project are £11.61 an hour for joiners and £8.73 for labourers. Employees are also entitled to increase overtime rates if more than 39 hours are worked in a week and should receive subsistence allowances if living away from home.

Mr Findlay has now raised a motion in Holyrood calling for the Scottish Government to begin a pay audit on the entire project.

While the Forth Crossing Bridge Company (FCBC) has denied the claims, UCATT said it has offered no evidence to support these assertions.

The union has also written to Keith Brown MSP, the minister responsible for the project, calling for an independent pay audit to be carried out.

Steve Dillon, regional secretary for Scotland at UCATT, said: “The Forth Crossing should be the jewel in the crown of the Scottish Government but it risks having its reputation permanently tarnished as Scrooge bosses are being given free rein to exploit workers.

“In order to restore confidence in the project the Scottish Government needs to urgently undertake an independent pay audit into the project which doesn’t just examine what workers are currently receiving but what companies have previously paid on the project.”

Mr Dillon added: “This country prides itself on common sense and fairness – so now the Scottish Government needs to show some. We welcome Neil Findlay’s motion and now call on the Scottish Government to audit this listing project before it starts sinking under a tide of infamy.”

Since construction began, the £1.4 billion crossing has not been without its problems. In 2015 workers downed tools over unpaid holiday pay and last month a raid by immigration officials found seven workers from the Indian sub-continent being employed illegally.