The agency represents both companies as part of the brief, having been Park Resorts’ retained agency since the start of this year.
Instinctif has been offering strategic counsel and media relations for the proposed merger, which was announced earlier this week and is subject to clearance by the Competition and Markets Authority.
If approved, the newly enlarged company will operate 73 parks across the country and serving 1.8m customers each year. It will have a turnover of £370m and underlying earnings of about £92m.
Instinctif previously advised Parkdean on its flotation in 2001 and its sale to Alchemy Partners in 2006. Parkdean's sites are predominantly located in the south west and Scotland, while Park Resorts’ estate centres on the east coast and north west, including the Lake District.
The proposed sale follows that of another major holiday park group, Centre Parcs, to Brookfield Property Partners in June.