The site-wide deal described as ‘best in class’ by all signatories sets pay and conditions for the project, which will cover up to 8,000 workers at peak construction.

Details have still to emerge, but the new deal is said to offer better sick pay and parental leave terms than agreed in 2013 for the Hinkley Point C site.

The three Solidarity Agreements set out how Sizewell C will promote a strong safety culture while providing workers with  high standards of health, safety and welfare services, including mental health services.

The deal comes as hopes grow that the Government can achieve a final investment decision before a likely election in the Autumn.

Firms that have signed include:

The agreement aims to raise the bar on standards of accommodation and promote a diverse and inclusive working environment.

Sizewell C’s joint managing director, Nigel Cann, said: “Sizewell C is one of the most important energy projects in the UK and we want it to be one of the best places to work.

“These balanced agreements will help build a spirit of partnership by giving everyone on the project common goals and clear working arrangements. They put us in an even stronger position as we gain momentum and prepare for site earthworks to start.”

Charlotte Brumpton-Childs, GMB National Officer, said: “These agreements represent some of the most progressive within the construction sector.”

Jerry Swain, Unite National Officer, said: “This agreement builds on the previous success at Hinkley Point C. It clearly sets out the standards for the future which other employers in this field should seek to achieve.”

Around 600 people are currently working at the Sizewell C construction site, and the workforce is expected to increase to around 1,500 by the end of 2024.

Recent activity has focused on clearing the site for full construction, carrying out archaeological surveys, and local mitigation work.

Preliminary earthworks to prepare for the construction of a concrete batching plant and water desalination facility are due to begin in the coming weeks.