The unions have called for staff at two of the contractor's subsidiary companies to be awarded the same pay and bonuses as those working for their parent. Laing O'Rourke's skilled tradesmen at T5 can make up to £55,000 a year.
Officials want to extend that deal to staff working for Select Plant Hire, the contractor's plant division, and Express Laing O'Rourke, a joint venture company that specialises in steel prefabrication.
John Gould, a regional officer with UCATT, said: "We are looking to include the workers so that they have parity of pay and also have the same kind of terms and conditions as the core workers on the project."
Gould added that he had discussed the issue with Laing O'Rourke officials and was expecting their response by today.
It is understood that an unofficial meeting of 40 drivers of Select Plant Hire was held last week.
The drivers are angry because they have not received the same deal as other T5 staff. Under the current scheme, the drivers are not reimbursed for the cost of lodging at the site, but work five shifts and get paid for six.
We are looking for workers to have parity of pay
John Gould, UCATT
The move comes as industrial relations on the project looked to be improving.
Laing O'Rourke's directly employed workers had been unhappy about their bonuses. They were also complaining that the company was not keeping its agreement to allow then to finish early one Friday a month so that they could travel home to see their families.
The unions negotiated a deal with the employer over these issues, and this was accepted by the workers at a mass meeting last Friday.
Under the terms of the deal, the employer made provision for bonus payments to be reviewed and offered workers who lodged at the site cheap air fares to many UK destinations. This will affect 1000 staff.