Engineer Motherwell Bridge has been brought in to work on fuel tanks at the Heathrow Terminal 5 project in west London.
The move may aggravate a dispute over pay among workers employed by McTay, the contractor hired originally to build fuel tanks and pipe system.
McTay’s workers have threatened to stop working overtime on the grounds that they are not receiving the same pay as Terminal 5 workers.
Airport operator BAA claims that although McTay is working within the terminal perimeter, it is not employed under a T5 contract.
The employment of Motherwell is not thought to be connected to the dispute.
A union source said a meeting was planned with contractor Laing Utilities over the employment conditions yesterday.
He said: “The workers are within the perimeter of T5 and are not earning the inflated T5 wages.”