Those involved in the dispute are all members of the M&E union Amicus, which is waiting until the results from other areas are in before making any specific decision on industrial action.
A total of 26 Watson steel workers voted in favour of strike action; nine voted against. Twenty-eight workers voted in favour of action short of strike, such as working to rule.
Members of the GMB are also voting over whether to take strike action. The dispute is over pay: steel workers have been offered 8% over two years, which they have rejected.
A T5 spokesperson said: "We are obviously disappointed that a small number – less than half of the 81 T5 workers affected by this national pay dispute – have voted for this action."
The spokesperson said that pay at T5 was often the best in the industry, and that industrial relations were generally good.
The spokesperson added: "The T5 programme allows for contingencies such as this and the opening date will be unaffected."
Amicus is balloting three other engineering construction sites in addition to Heathrow and Wembley. These include the BP site at Grangemouth, Falkirk; British Nuclear Group at Sellafield, Cumbria; and the Novartis Project at Grimsby, Lincolnshire.
A union insider said officials were waiting to see the results of the ballots on all the sites before making a decision on possible strike action. The insider said that early indications were that workers would want to take action.