Nestlé, the world's largest food and drink company, has put its £63m UK & Ireland media account into review.
Zenith has the KitKat and Nespresso brand owner's media planning and buying since 2014 (when it was known as ZenithOptimedia) and is understood to be defending the business.
The Publicis Groupe agency was invited to repitch alongside shops from other major ad networks.
Nestlé spent £63m in the UK last year, according to Nielsen estimates. The majority of its spend went on TV (£42m), followed by outdoor (£16m) and radio (£3m).
Campaign understands that Nestlé has held initial meetings with prospective agencies about their credentials and tech capabilities, ahead of drawing up a shortlist of agencies that will be invited to pitch.
A spokesman for Nestlé said the company would not comment.
Nestlé, a Swiss conglomerate, has been the world's largest food and drink producer for more than a decade, according to the Forbes Global 2000 report (apart from in 2018 when it was supplanted by AB InBev).