The refinery’s owners, Total, said: "We are extremely disappointed and frustrated that our contractor workforce on the HDS-3 construction project has decided not to return to work.
"The contractor workforce was asked to return to work and end the illegal walk-out at a meeting between contractor management and union representatives this morning. The contractor workforce decided to continue the unofficial action. The HDS-3 construction project will remain closed until everyone agrees to return to work. However, the refinery continues to operate normally. and is not affected by this action.
"We are keen to clarify the demands of the contractor workforce so we can resolve the situation as soon as possible."
Meanwhile, the GMB said it was considering legal action after a confidential letter to employers gave advice on how to deal with a nationwide ballot of engineering construction workers over pay, jobs and conditions.
