Thousands of engineering and construction workers at U.K. energy plants have voted to reject their employers' offer to improve pay and conditions and may begin industrial action, a leading union said Monday.

Up to 30,000 engineering and construction workers rejected the offer following individual ballots held over the past two weeks, the GMB union said.

The GMB union said workers at seven sites - Ineos PLC's 210,000-barrels-a-day Grangemouth refinery in Scotland, Royal Dutch Shell PLC's (RDSB.LN) 240,000-barrels-a-day Stanlow refinery, RWE AG's (RWE.XE) Aberthaw and Staythorpe power stations, Chevron Corp.'s (CVX) Pembroke Refinery in Wales and the Sellafield nuclear site in England - had now voted in favor of another round of industrial action.

In June, hundreds of laid-off workers at Total SA's 200,000-barrels-a-day Lindsey oil refinery staged protests and got their jobs backs after contractor companies at the site met demands.

The protest prompted copycat strikes at dozens of other sites across the country resulting in more than 8% of the engineering and construction industry's workforce downing tools.

The speed and scale of the wildcat strikes, unprecedented in the U.K., caught energy companies by surprise, with the protests organized by assembling large groups of people at specific sites using mass text messages and social networking Web sites. It is a phenomenon that could easily be recreated this winter if workers decide to strike again, according to industry analysts.

"The members want the package to be completed now so that they can see what they are getting. The next step is to go back to the employers to see if they are up for further talks," Phil Davies, GMB national secretary, said.

The GMB is seeking, among other demands, a national skills register and a procedure to ensure all workers have appropriate skills, plus a register of unemployed workers in the industry which employers must use to fill vacancies.

The unions are also looking for a pay rise for workers, in addition to 12 paid trips home for all workers regardless of where they come from, plus the ability for full-time union officials to instigate grievance procedures at any site.

-By Angela Henshall, Dow Jones Newswires; +44 (0)20 7842 9285; angela.henshall@dowjones.com