Workers at confectionery giant Mars fear they may face redundancy following speculation that some production may be moved overseas.
There are concerns that cuts will be made at the main UK factory in Slough, Berkshire, if the move goes ahead.
The plant employs about 1,500 people and produces products such as the Mars bar, Maltesers, Twix and Snickers.
Work will halt at the plant on Thursday for the first time since 1932, when the company will announce its plans.
Frank Loveday, regional officer of the Bakers, Food and Allied Workers' Union, said workers feared job cuts and changes to hours and shift patterns but had not been given any firm indication of what was going to be announced.
Mr Loveday said: "The people who will do the briefings will not be told until an hour beforehand what the announcement will say.
"It is obviously a major announcement - they won't be discussing changes to the canteen facilities."