The snacks company, which owns brands such as Jacob's and McVitie's, has moved the retained UK trade brief into a sizeable 3 Monkeys account that already includes corporate and consumer work.
The move, which follows 3 Monkeys being hired in December and the arrival of United Biscuits comms lead Rebecca Salt in April, ends a relationship thought to span nearly a decade.
Salt said: "Our business has evolved significantly over the past 12 months, and as a result we have decided to consolidate our communication support.
"3 Monkeys Communications has proven it understands our business since its appointment at the start of the year, so it makes sense to integrate the trade PR work into its remit. We have enjoyed working with Publicasity and we wish it well in future endeavours."
3 Monkeys MD Christine Jewell added that the consolidation would help the agency "tell the full story" of United Biscuits.
Publicasity has not yet responded to a request for comment.