A consultative ballot on possible industrial action by Hinkley C power station workers is to be held by the Unite and GMB unions.
They claim contractors failed to pay a predicted bonus and warned industrial action could lead to "expensive" delays at the Somerset nuclear plant site.
About 700 members of the workforce are expected to be balloted.
Work began on Hinkley Point C last month. It is expected to begin generating electricity in 2025.
Regional officer Tim Morris said: "Our members came to Hinkley Point under the belief that they would receive excellent pay and bonuses.
"The employers think they can do this on the cheap by offering a derisory bonus rate, but the workers consider it to be completely inadequate to attract and hold on to the skilled workforce necessary."
EDF Energy, which is behind the scheme, said: "We are committed to a continuing dialogue on this issue with our contractor and trade union partners.
"Discussions are ongoing and our intention is to reach a mutually acceptable outcome."
The ballot is scheduled for May 2-5, the unions said.