John Butler, the HTW chief executive, has left as a result of the move. Trish Wheaton, the president of Wunderman Canada, has taken over as the chief operating officer of Wunderman in the UK.
A chief executive and executive creative director will be named in the next few months, but in the interim, Marcus Starke, the head of Wunderman EMEA, will take an active role in the UK business.
Butler's departure comes just one week after the agency scooped Orange's £18 million consolidated direct marketing account after a four-way shoot-out against Chemistry and the incumbents, WWAV Rapp Collins and Craik Jones Watson Mitchell Voelkel.
The Orange win was a rare piece of good news for HTW, which has lost a number of big accounts recently, including Vodafone and Microsoft.
Butler's exit follows that of Steve Harrison, the worldwide creative director and co-founder of HTW, who left two months ago without a job to go to. Fellow co-founder Martin Troughton left in 2004. The joint managing directors, Suzanne Partridge and John Hiney, departed last year.
Wunderman Automotive's existing clients include Ford, Volvo and Mazda.