The fear is that this sectional dispute, which involves steel erectors and welders, could spread. If electricians, a traditionally militant group, refuse to cross union picket lines, the situation could deteriorate rapidly.
Negotiations between the M&E union Amicus, which represents the steel erectors, and the employers became deadlocked some weeks ago and the union believes the only way to resolve the dispute is through industrial pressure.
The employers in the steel erectors' dispute said that their pay offer of 8% over two years was reasonable. The statement said: "This offer was, is, and will remain, final. In the current climate, it is the most the industry can afford."
Site activists at a number of other high profile projects, including Wembley stadium in north-west London, have also been pressing for strike ballots. The union held a meeting on Monday at the Wembley site, attended by workers belonging to steel contractor Cleveland Bridge.
Brian Rogan, the managing director of Cleveland Bridge, said he was waiting for clarification from Amicus over whether the Wembley site would be balloted.
I am concerned the electrical workers could breach the terms of their contracts
Amicus official Frank Westerman
He said: "Wembley was not a site that was originally going to be balloted; we will wait to see if that situation changes."
Amicus intends to ballot three other engineering construction sites in addition to Heathrow Terminal 5 and Wembley. These are: the BP site at Grangemouth, Falkirk; British Nuclear Group at Sellafield, Cumbria; and the Novartis project at Grimsby, Lincolnshire.
There is increasing anxiety among employers and BAA, the client at Heathrow Terminal 5 client, that this dispute could escalate if electrical workers become embroiled. They are covered by a separate agreement but are unlikely to cross a picket line, even though this form of action is illegal.
Frank Westerman, Amicus' regional organiser, said: "I am very concerned that this situation could develop and that the electrical workers could breach the terms of their employment contracts."
He said that steel erectors and electrical workers at Heathrow were due to meet on Wednesday, when the union aims to set up a branch representing both groups.
BAA said in a statement: "The pay arrangements at T5 across all trades are some of the best in the industry. We are optimistic that industrial action will be avoided, but we have contingency plans in place for a range of eventualities.
The London electricians: a tradition of union militancyFears over union militancy in the electrical sector go back to the late 1990s when unofficial strikes broke out on the Jubilee Line extension underground project in a row over pay.
More than 1500 electricians protested over a deal negotiated by their own unions that involved a pay rise of nearly 20% over two years.
Electricians said that they did not receive pension payments and needed to earn big money in boom times to make up for long periods of recession.
This year M&E union Amicus negotiated a pay and conditions deal with employer and BAA on the Heathrow Terminal 5 project that could see electricians earning more than £50,000 a year.
Pressure had been building on the M&E sector after another union, UCATT, brokered a much publicised deal with civils contractor Laing O’Rourke that enabled construction workers to earn £55,000 a year.