Young’s Seafood has lost a £30m contract with the supermarket chain Asda.
A spokesman for the Grimsby firm said it was disappointing news but Asda remains an important client.
The company also highlighted recent contract wins, which it said demonstrated its ability to retain and grow the business.
Asda, which said it would not comment on individual contracts, said Young continued to be an important partner.
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According to the trade journal Undercurrent News, Asda has given Young notice of around £30m business-and-white shrimp.
It was reported the prawn of work would be heading to France, while the white fish contract would go to Albert Darnell, also based in Grimsby.
Tom Seaman, editor of Undercurrent News, said: “The uk seafood sector is going through a very difficult time with the high cost of inflation after Brexit and the currency situation.
“Retailers are under pressure and that is causing this reset in the contracts and can take a long time to settle on the supply of that in the future”.
In April, Young’s Seafood, which employs about 1,700 people in Grimsby, was put up for sale.
The company has been in business for over 200 years and is based in the north Lincolnshire port for more than 50 years.
Young has also confirmed the closure of its Pinneys of Scotland plant of Annan, which came after the consultation on the proposals to move production of the Scots site at Grimsby.