In a three-hour meeting on Wednesday morning, the contractor was warned that it faced industrial unrest unless agreement was reached over improved bonus payments and working hours.
The dispute involves three unions, UCATT, the T&G and the GMB, which between them have more than 1500 members working on the £3.7bn project.
In addition to increased bonuses, the unions want the contractor to give some workers extra time off so that they can travel home for long weekends once a month.
The unions are to hold another meeting on Tuesday. If there is no agreement with Laing O'Rourke, they will discuss with their members whether to proceed to some form of industrial action.
Phil Davies, GMB national construction officer, said: "We have laid down a series of proposals to Laing O'Rourke and we will meet on Tuesday, hopefully to resolve the issue."
It is understood, however, that the GMB is taking a harder line than other unions.
An insider said that the GMB is ready to move to a ballot on industrial action if agreement is not reached, and has drawn up a consultation paper setting out the arguments in favour of an aggressive response.
We have laid down a series of proposals to Laing O’Rourke and will meet on Tuesday
Phil Davies, GMB national construction officer
The insider added that the GMB's hard line was motivated by a desire to increase its membership on the site. UCATT and T&G would be more inclined to continue negotiations.
An insider at the GMB denied that this was the case. He said that the dispute had been going on for some time and that was why the union was becoming more militant.
In addition to increased bonus payments the unions want workers who live long distances from Heathrow, and live on site during the week, to be allowed to leave on a Thursday night so they have one Friday off each month.
Under an agreement negotiated in March, it was agreed that workers would leave the site at lunchtime on Friday once every five weeks.
T5 made national headlines in January this year when skilled workers with Laing O'Rourke were awarded a £55,000 pay deal.
The escalation in the dispute comes in the same week that the GMB, Amicus and the T&G held lengthy talks with BA over the introduction of a swipe card system for airport workers.