Trade unions for construction workers at the Hinkley Point C project in Somerset are to hold a ballot for strike action in a dispute over pay.
It comes as more than 95% of GMB and Unite members voted against the proposed pay increase offered by French utility EDF and its contractor BYLOR.
The dispute, which has been ongoing since the spring, concerns the pay of workers engaged on civil engineering contracts at the proposed nuclear power station.
A ballot for strike action was called off in June after an interim agreement on bonus payments was agreed.
The three-month agreement was extended into September, while both sides hoped to reach a permanent agreement.
The unions say the pay rates for workers on civil engineering contracts are “significantly below” the rates of those covered by the mechanical and engineering (M&E) contract.
EDF Energy said it was “disappointed” and that it had offered increased pay, bonuses and benefits as part of a “fair and reasonable” package.