More than 2,000 engineering construction workers in the Unite and GMB unions at seven petrochemical plants and power stations are balloting for an official strike over jobs.
The ballot follows months of unofficial action across the country.
The ballot ends on Tuesday 1 September.
The seven sites are Ineos's Grangemouth oil refinery in Scotland, Shell's Stanlow refinery in Cheshire, BP's North Sea oil processing terminal at Grangemouth, Sellafield nuclear plant in Cumbria, RWE's power stations at Staythorpe in Nottinghamshire, Aberthaw in South Wales, and Chevron's refinery in Pembroke, west Wales.