But Mr Campbell's move has in turn prompted a call for him to quit his First Milk post too, amid claims that his action is an embarassment to the organisation.
For his part, Mr Campbell claimed his decision was a family one. In a statement to The Scottish Farmer, he said: "Having mulled things over for a few months, my family made the decision to resign our membership.
"I've talked through the specific reasons for resigning and suggested improvements with Nigel Miller, and was really pleased by the constructive conversation we had.
"While I will keep the detail of that conversation private, I will continue to work closely with Nigel and others at the union in my representative role at First Milk."
First Milk's communications director, Paul Flanagan, commented: "Tom's family has made a decision to resign its membership.The relationship between First Milk and the union is unaffected."
(For the fully story see The Scottish Farmer this week, February 2, 2013)