His departure is said to have been agreed with the Muller management in a meeting last week, though a company spokesperson denied the story.
A revised company statement issued yesterday (Wednesday) confirmed he would leave on 30 June.
Harrison, who took the role in January last year, was responsible for the review that saw TBWA\London scoop the £14 million Muller ad account in October last year.
The agency was awarded the business following a four-way pitch between Abbott Mead Vickers BBDO, HHCL/Red Cell and M&C Saatchi.
Publicis, the incumbent on the account for five years, was not asked to repitch for the business. TBWA's first work for the company broke in January this year and used the new tagline "lead a Muller life", aimed at encouraging people to eat healthier foods.
In March this year, Harrison also handed M&C Saatchi a £2 million brief to work on Muller Desserts. He worked with M&C during his time as the marketing chief at Nestle.
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