Following a series of pay freezes and below-inflation pay rises, industrial tensions had increased across the 7,000-strong workforce.
Under the deal between unions and employers in the National Agreement for Engineering Construction Industry (NAECI) the extra cost of living rate will apply from now until 30 June.
After that date, the supplementary payment will become 75 pence an hour.
Unite general secretary Sharon Graham said: “This is a significant increase in pay for workers, who have benefited from making a collective stand and forced their employers to take action on the cost of living crisis.”
Despite the acceptance of the supplementary payment, local disputes over bonus payments involving NAECI workers, including at Drax, Grangemouth, Torness, and Mossmoran, will continue, as workers continue to seek a further increase in bonus payments, she warned.
The ongoing local action will put additional pressure on employers to resolve the enduring tensions with the agreement when talks begin later this year for a new pay agreement, which is due to come into effect in January 2024.