Harrison, who is understood to be taking a break from the industry, founded HTW six years ago, following the acquisition of his hotshop HPT by Impiric, the Wunderman-owned agency.
Considered one of the world's most accomplished DM creatives, Harrison won the inaugural Cannes Lion Direct Grand Prix, as well as three other gold, five silver and two bronze Lions for accounts including IBM, Star Alliance and Microsoft.
Harrison's departure comes after a management restructure at Wunderman. In February 2006, Marcus Stark was made the head of Wunderman EMEA and, in June, John Butler was hired as the chief executive of HTW London.
Since then, the agency has lost several of its blue-chip clients. Vodafone, M&G, Star Alliance and Microsoft have all moved elsewhere.
HTW has also lost several key members of staff in the past few months, including the joint managing directors John Hiney and Suzanne Partridge, who left in October last year, as well as the business directors David Mobbs, Giles Horton and Howard Geisler.