Steve Parish, chief executive of Tag Worldwide, will step down at the end of this calendar year to pursue "other interests outside of the business".
Parish, who sold international design and production agency Tag to Williams Lea in July 2011, will be joined in his exit by fellow Tag shareholder and commercial director Richard Jameson, who will also step down at the end of the year.
In a statement announcing his departure, Parish said: "My two decades at Tag have been an incredible period and I am proud of what we have achieved.
"At the end of a first year of integration with Williams Lea, it feels like the right time for me to move on and pursue my other interests outside of the business."
Williams Lea group chief executive Conor Davey, who took over from Tim Griffiths in November 2011, said: "Whilst we will miss Steve's energy and passion on the board, we respect his decision to leave and are full of admiration for the business he has built, which is now very much a central part of our growth strategy."
Williams Lea declined to comment on who would head up Tag following the departure of Parish and Jameson, other than to say that it would be brought together with its Marketing Solutions business "to offer clients a wider range of services to improve their marketing efficiency".Tweet