The union Unite is to ballot workers at Cadbury's about taking industrial action over the closure of a factory.
The closure will be phased in from 2009
About 500 jobs will go when the plant at Keynsham closes from 2009 onwards.
More than 100 workers from the plant will join other union members to protest about the cuts at a rally in London on Wednesday.
Unions and managers have already met to discuss the closure and the future of the Bournville plant in Birmingham, where 200 jobs are under threat.
Brian Revell, of Unite, said: "Cadbury's management were left in no doubt of the depth of feeling in the plants and in the local communities affected."
A protest rally against the plans, involving about 200 people, was held earlier in October.
The Keynsham production will be moved to factories in Poland and Bournville as part of a wider cost reduction plan announced by Cadbury in June.