Wunderman Automotive is responsible for the direct marketing and digital accounts of Ford, Land Rovver, Mazda and Volvo in the UK.
The new agency will be called Wunderman and it is understood that John Butler, HTW chief executive officer, will leave the agency, with Trish Wheaton, president of Wunderman Canada, becoming chief operating officer.
Wunderman confirmed that a new chief executive officer and executive creative director would be named in the coming months. During the transitional period, Marcus Starke, head of Wunderman EMEA, will be active in the UK business, with Alexei Orlov, worldwide vice-chairman, to take responsibility for the global automotive group.
Wunderman said the newly formed agency will now operate as one entity, and would create the largest data, digital and relationship marketing company in the UK.
Starke said: "This strengthens Wunderman's value proposition to our clients in the UK and across the entire network," and would create "the strongest digital marketing services in the world".
Harrison, who founded HTW in 2001, became worldwide creative director in March 2006 and expanded his role within the company. His departure occurred after a management restructure in February last year, which made Marcus Starke head of Wunderman EMEA.
His leaving came after a spate of senior departures at the business, including joint managing directors John Hiney and Suzanne Partridge, who left in October last year, and business directors David Mobbs, Giles Horton and Howard Geisler.