Cadbury has moved its £100 million global advertising account out of Publicis and into Saatchi & Saatchi and Fallon's SSF Group.
Although the Trident account, held by JWT, is currently unaffected, insiders say it would be surprising if all non-Publicis-aligned business did not move to SSF as part of a broader consolidation.
Chris Palengat, the Publicis Groupe worldwide account director on Cadbury, and the rest of the Cadbury team at Publicis are expected to move across to SSF.
In London, it is thought that SSF's first tasks will be for Wispa, as well as some digital briefs for other brands.
The move, under discussion for months, is understood to be an attempt by Maurice Levy, the Publicis Groupe chairman and chief executive, to bolster the fortunes of SSF, which, until now, has not won a piece of group business.
Levy is thought to have persuaded Cadbury that a move to "tidy-up" the confectionery business would be in the interests of both parties.
One insider attributes the move to SSF to Cadbury being "seduced by Fallon and Kevin Roberts".
SSF is headed by Roberts, the Saatchi & Saatchi worldwide chief executive, and Robert Senior, the founding partner of Fallon and chief executive of SSF in the UK.
The decision will be a blow to Publicis' London agency, which has produced some of its most high-profile creative work for Cadbury brands including Creme Egg and Wispa.
However, the account has been under threat since 2006, when Fallon won the Bournville business. In 2007, it snatched Dairy Milk.