Brian, who took over from Paul Stoddart earlier this year following a company restructure, is not believed to have a new job to go to.
Brian's responsibilities, which covered Euro RSCG Skybridge and its employment marketing sister arm Euro RSCG Riley, will be taken on by group chief executive Rupert Grose.
Other changes at the business include the promotion of head of learning and development Julia Jawnyj to managing director of performance improvement. She will work closely with Euro RSCG head of events Lori Stahl.
Grose said: "Having developed a strong team around him, (Brian) felt confident that it was the right time to move on and pursue other business interests closer to home in the Midlands."
Jim Tudor, a director at sister agency Euro RSCG Heist, has also left the group, although the agency will continue to collaborate with him on a number of projects.