A mass meeting of workers employed on the Terminal 5 site rejected an offer of a 22p an hour increase, backdated to June.
An official strike ballot will now be held, with the result due next month.
Tom Kelly of the GMB, said he was confident workers will vote in favour of a campaign of strikes.
"The company will need to be ready to reopen talks and to be prepared to enter into meaningful negotiations if they want to avert a stoppage."
Alan Brkljac, of the Transport and General Workers Union, said: "This categoric rejection strongly increases the likelihood of a full strike ballot, in fact I would say it is a certainty."
News stories provided by third parties are not edited by "Site Publication" staff. For suggestions and comments, please click the Contact link at the bottom of this page.