The call forms part of a pay and conditions claim put forward by the Unite and GMB unions under the National Agreement for the Engineering Construction Industry (NAECI).

The latest NAECI claim for “2016 and beyond” calls for wide-reaching reforms of the sector following growing unrest among the workforce.

The unions warn of “the threat to stable labour relations in this important industry in delivering the next generation of low-carbon energy projects on which the rest of the UK economy is dependent.”

They are now demanding a “substantial” pay increase and a reduction in the current working week from 38 hours to 34 hours.

The long-term goal is a move to a four-day working week phased in over a ten-year period with “no loss in earnings at the time of the phased reduction”.

The demands have shocked employers – many of whom operate in the petro-chemical sector and are still reeling from the recent fall in oil prices.

Uncertainty also still surrounds the future of new nuclear power projects.

One industry source told the Enquirer: “There are huge doubts over the finances of new nuclear builds and the petro-chemical sector is reeling from oil price falls so this is the last thing anyone needs.”

The employers said the combined demands of the claim could add 20% to industry costs.

The Engineering Construction Industry Association said: “Looking into 2016, no upturn is visible in engineering construction.”

Both sides are set to hold further talks next week.