Nearly 500 jobs are being cut by food producer Birds Eye with the closure of its fish foods range factory in Hull.
The Hull plant was founded in 1967
In a statement the company blamed what it called unsustainable capacity in its supply chain.
It said its fish foods range would now be produced at sites in Bremerhaven, Germany, and Lowestoft, Suffolk.
The company said 490 redundancies were involved, but the GMB union said it understood 600 jobs would be lost and described the announcement as "grim".
John Wilson, the union's senior organiser, said: "There's the knock-on effect that also needs to be considered and those working for supply companies will also be affected.
"The only compliment I can give this company is that at least they waited until after Christmas for this announcement.
"It's going to be very difficult for people to find new jobs in this area as the manufacturing industry in Humberside has been going through a difficult time. Any jobs they are forced to take will be at minimum wage and with minimum holiday and other benefits."
Employees were given the news during a morning meeting with chief executive Martin Glenn at the factory.
The factory produces foods such as fish fingers, fish cakes and peas
Mr Glenn said: "We have inherited a business that has unsustainable over-capacity. The frozen food market is increasingly cost-driven.
"To be competitive you require large scale, highly efficient production facilities.
"The future success of our fish category can be best served through the consolidation of our production facilities into our larger and more efficient sites in Bremerhaven in Germany and Lowestoft."
Factory manager Andy Stark said: "Clearly, this is devastating news for the people in the Hull operation.
"We have a highly committed and skilled workforce who have tried their very best to move the site forward over the last couple of years in an attempt to retain volume at the factory."
Ian Kelly, the chief executive of Hull and Humber Chamber of Commerce, said: "This is very sad news. These are skilled and talented workers who have made a good profit for their business over the years and we hope a new buyer can be found."
Mr Wilson said union officials would be going in to the factory on Thursday to negotiate redundancy packages for the workers.
The factory, in west Hull, is in the constituency of Education Secretary Alan Johnson. A spokeswoman for the MP said he was being briefed about the situation and was expected to comment later.
The Birds Eye company employs 1,250 people at sites across the UK.
The huge processing plant in Hull was established in 1967 on a 17-acre site.
It has seven main production lines turning out foods such as fish fingers, fish cakes and peas.
Unilever, which owned Birds Eye until August last year, closed its Grimsby plant in 2004/5.