Mustard maker Colman's is to leave its base in Norwich where the condiment has been produced for 160 years.
Colman's, which employs 113 people at its site in the city, will move its production to Burton-on-Trent and Germany.
The GMB union described the closure as "absolutely devastating news".
The union said 43 jobs would be transferred to the Burton site, with 20 moving to a new facility opening in Norwich for milling mustard seeds.
Fears of a closure had been raised when Britvic, who shares the site in Carrow with Colman's, announced it was leaving last October and transferring its production of Robinsons and Fruit Shoot to elsewhere in the UK.
The company said in December it had reviewed its operations in light of Britvic's closure proposal.
Colman's, which has been making mustard at the site since 1858, became part of Unilever's Van Den Bergh Foods in 1995.
Unilever said it would "protect the historic link between Colman's and Norwich" by retaining the "historic mustard milling process" and other production in "a new state-of-the-art facility in the Norwich area".
"We recognise that these proposals will be difficult news for our Norwich employees, their families, and the whole community in Norwich," a spokesman for the company said.
"We are committed to supporting our employees during this difficult time."
Norwich South MP Clive Lewis described recent weeks as "a truly awful festive period for hundreds of Norwich food manufacturing workers and their families".