It is understood that Venn, 50, announced his resignation just before Christmas. It is the second significant departure from F+A in seven months. Former managing director Kevin Arnold left for Gardiner & Theobald in September.
The firm merged with Mott MacDonald two years ago, creating a 7000-strong, £400m-turnover multidisciplinary business. However, some observers have suggested the move led to more of an emphasis on engineering than property.
Venn worked for more than 20 years at City practice Watkins Pool before going to F+A.