The GMB and Unite unions claim the Bouygues/Laing O’Rourke joint venture overseeing main construction has failed to pay the bonus rate predicated in the site industrial relations agreement.

An industrial relations flare-up so early in the project is a set back for the HPC Civil Construction agreement, which set out a clear framework to avoid costly disputes.

The first concrete pours for the main station foundations only just got underway at the end of March.

Now unions plan to hold a consultative ballot of the 700-strong workforce preparing the groundworks at the Somerset site. The ballot will run between 2-5 May.

Union officials argue that the bonus rate offered by the BYLOR consortium is insufficient to attract the quality of workers needed to ensure that that the civil works phase of the £18bn project is completed on time.

The Enquirer revealed earlier this month that construction workers are aiming for £60,000 packages on site, but it is understood current offers are a long way south of this figure.

Unite regional officer and chair of the HPC joint union committee Rob Miguel said: “The project has already suffered delays and to achieve the 2025 start date for electricity generation a skilled construction workforce is required.

“If the consultative ballot eventually leads to full-scale industrial action ballot, we could be looking at delays to the construction at Hinkley Point, which will be very expensive for the employers as hired-in plant and machinery will be lying idle.

“We would urge the employers to get back around the table with senior union officials to achieve a fair settlement, so building can continue to the highest standard, safely and on schedule.”

An EDF spokeswoman 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.

“We have a strong partnership in place with the trade unions based on a set of landmark agreements that include pay and productivity.

“We share a commitment with them and our contract partners to provide high quality employment standards, alongside the best standard in health, safety, quality and productivity.”