Steven Deans, the Unite union's representative at the Grangemouth refinery in Scotland, was at the centre of a dispute which led to the refinery's bosses threatening to shut down the whole site.
He has now resigned a day before he was due to meet his employer, Ineos.
Mr Deans was also chairman of the Labour party in nearby Falkirk, and was accused of trying to install Unite's choice as Labour candidate for MP. ITV News Deputy Political Editor Chris Ship reports:
Grangemouth owners Ineos are in last ditch talks with the workers' union, but the workforce is just one of the problems facing the plant.
Thousands of jobs are under threat after a dispute over conditions led to a shock decision to close the petrochemical site in Grangemouth.