Unions are angry that within the negotiations Laing O'Rourke has refused to give a commitment that its subcontractors will hire only directly employed labour.
A BAA spokesperson said the project intended to use directly employed labour but it would not rule out using agency labour at some point.
He said: "The reality is that this project is going to last six years and it is our aspiration to have a directly employed workforce. But if somewhere down the line we need to bring in agency workers to complete a project then so be it."
If down the line we need agency workers, then so be it
Two meetings have already been held this month between union officials and Laing O'Rourke representatives. The firm is understood to be aiming for a deal that excludes national union agreements stipulating basic pay.
It is also understood to be negotiating a staff pay deal based on shift payment and bonuses, with renegotiated lodging and travel allowances. Laing O'Rourke refused to comment.