The acceptance of the deal comes after a drawn out and bitter dispute between operator Ineos and the Unite union over the dismissal of a union representative.

Ineos Grangemouth (UK) said that "almost the entire workforce" of 1,350 employees had agreed to the new arrangements. The deal puts the company in a good position to bring in significant new investment, Calum MacLean, chairman of Ineos Grangemouth (UK), said.

"With our costs coming under control, the shareholders are committed to making good on their promise of a £300 million investment, which will allow us to build a new terminal and use U.S. shale gas as a new raw material for the petrochemicals site," he said.

Ineos is the full owner of the Grangemouth petrochemical plant and a joint owner of the 210,000 barrels-per-day (bpd) refinery along with PetroChina, which holds 49.9 percent.

The company halted operations at Grangemouth in October and demanded changes in terms and conditions before it would permit a restart. It had previously said that losses would force it to shut the petrochemical plant.

It is now planning to double the number of apprentices and new graduate recruits it is hiring over the next three years.

(Reporting by Claire Milhench, editing by William Hardy)