Kerry Foods is planning to close its factory in the UK town of Burton-on-Trent - a move that will lead to 900 job losses.
The news has been described by union officials as a "crushing blow" for the area.
Unite said it would leave no stone unturned in helping its members affected by the announcement, and has already been in touch with the town's second largest employer regarding employment opportunities.
Regional officer Rick Coyle said: "This is a calamity for Burton as Kerry Foods is the town's biggest employer.
"This is a very sad day for Burton and the end of an era."
Unite said the closure was a result of Kerry Foods losing its contract to supply supermarket giant Tesco with ready-made meals.
Mr Coyle added: "The problem going forward is that there are not that many well-paid jobs in Burton and the vicinity to replace those that will be lost at the end of August.
"We have been in touch with Molson Coors (the town's second biggest employer) to explore employment opportunities there.
The union said its staff will meet management on Friday to discuss an enhanced redundancy package for its members.
- Press Association