The Birds Eye frozen food factory in Grimsby, North East Lincolnshire, is to close with the loss of 600 jobs.
The decision follows a year-long review of operations in Grimsby, Hull and Lowestoft by parent group Unilever.
News that the Grimsby plant will close early next year was broken to staff at a meeting on Thursday morning.
The GMB’s Andy Worth said: “It is a disgraceful and ridiculous decision that will put the livelihoods of 600 people in Grimsby in serious jeopardy.”
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James Hill, chairman of Unilever’s ice cream and frozen food division, told the BBC: “It is a very difficult day for everybody and emotions are understandably running high.
“I would just like to say what tremendous sympathy I have for the Grimsby workforce and express my commitment to do everything possible to mitigate the effects of the change.”