The owners of the Colman's factory in Norwich say the site will close at the end of next year.
Unilever say the first phase of consultation staff has been completed.
Production will start to transfer away from Norwich to the Unilever plant at Burton on Trent in autumn this year.
113 jobs in Norwich are now at risk of redundancy, around 40 will transfer to Burton.
Unilever says they want to keep production and packing of Colman’s mustard powder, the mustard milling process and mint processing in a new state-of-the-art milling facility in the Norwich area employing around 25 people.
No announcement has been made as to where that will be. The company is also pledging to source the mustard locally.