Five-and-a-half-year incumbent Ketchum has been axed from the account and replaced by Hill & Knowlton for the Pepsi, Diet Pepsi and Pepsi Max brands.
The sugar-free attributes of Pepsi Max will receive far greater focus.
The decision to switch the account – believed to carry annual fees of around £350,000 – has been made jointly by Pepsi and Britvic, which markets the Pepsi brand in the UK.
It comes as soft drinks manufacturers face up to a decline in consumer taste for fizzy drinks and a government clampdown on marketing to kids.
Britvic director of corporate affairs David Lewis said: "We want to be quite provocative but in a constructive way. We plan to demonstrate through H&K that we all have a role to play in debates on issues such as obesity."
Britvic's share price plunged last week when its CEO Paul Moody described the decline in its carbonated drinks market as 'severe'.
H&K UK marcoms practice head Richard Millar is leading the account.
The account switch comes less than a year after marcoms MD Bronwen Andrews left Ketchum – where she worked on the Pepsi business – to join H&K.
H&K already works for Britvic on its Robinson's brand. Freud Communications continues to advise Pepsi on UK and international corporate PR.
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