If no deal can be struck before the closure up to 1,200 workers will walk out.
From Monday production will be hit as Grangemouth operators Ineos close the first of the crude oil units as part of a phased shutdown of the refinery.
The 200,000 barrels per day refinery is integrated with a large petrochemicals plant.
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Ineos Communications Manager Richard Longden said the shutdown was starting on the petrochemicals side and moving to the refinery.
Drivers have been urged not to panic buy at forecourts.
Phil McNulty national officer of Unite has accused Ineos of scarmongering about petrol shortages in Scotland and the North of England.
He said: "The company appears to be inundating the press with stories about imminent petrol shortages, but the fact is that the refinery is still in full production and there is no industrial action going on."
The First Minister has urged both sides to talk ahead of the strike. Contingency measures are being put in place to secure fuel supplies and emergency services, local authorites and key agencies have to asked to get together to discuss needs at a local level over the next two days.
(GK/JM)
