The switch is part of efforts to make the Kingsmill brand a more potent competitor to its rivals, Hovis and Warburtons, and stave off the threat from supermarkets' own labels.
Industry sources confirmed M&C Saatchi had been picked to take over the business without a formal pitch.
The appointment extends M&C Saatchi's relationship with Associated British Foods, the Allied Bakeries parent, which began in Australia seven years ago.
Associated British Foods' George Weston Foods subsidiary owns Tip Top Bakeries, whose Mighty White brand is handled by M&C Saatchi's Sydney office.
Brian Robinson, the New Zealander who became the chief executive of Allied Bakeries at the end of 2005, previously worked with M&C Saatchi in Australia.
JWT took over Kingsmill from Publicis in March 2004. Last month, JWT launched a new national TV campaign to support the introduction of Kingsmill's wholegrain and white bread.
Kingsmill, Hovis and Warbutons each have a share of between 14% and 17% of the UK bread market, while the combined supermarkets' own labels account for about 35%.
Neither the agency nor the client were available to comment on the news.
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