The ballot on the latest improved offer of 10% for 2024 and a further 5% for 2025 closed Friday and saw a majority of 86% turn the deal down.

GMB and Unite officials will meet with union reps today to discuss next steps and a strategy for industrial action.

Charlotte Brumpton-Childs, GMB National Officer, said: “Engineering construction worker’s pay has plummeted to more than 20% below inflation.

“So it’s not hugely surprising they haven’t accepted a pay deal that still amounts to a real terms pay cut.

“These are extremely skilled workers, who are considering mass legal industrial action for the first time in their careers because their employers have failed to keep their rates of pay in line with inflation.”

Action by workers covered by the National Agreement for the Engineering Construction Industry (NAECI) could hit the Stanlow, Fawley, Valero, Grangemouth and Mossmorran Oil Refineries, as well as at the Sellafield nuclear facility.

Unite national officer Jason Poulter said: “There is a limited window of opportunity for NAECI contractors and clients to avoid widespread industrial unrest.

“They have the money to ensure that our members’ rates are restored to their previous value. Their latest offer was inadequate and was overwhelmingly rejected by our members. They need to get back into negotiations and work with us to find an acceptable offer.”