The make-over will be the first for Jaffa Cakes for nearly ten years, when Design Bridge devised the iconic 'orange peel' typography for the logo, and it is understood that McVitie's feels it is time to refresh the brand.
WMH won the business after a four-way pitch in June last year and is thought to have come up with a significant idea for the brand.
According to one source, the brief was "very open and very strategic".
However, McVitie's deputy managing director Colin Hutchison sought to play down the appointment, saying that McVitie's "has a big programme this year and this is just one project".
He said the make-over, which will not constitute a name change, is still being developed.
Although one source said the packs could be on-shelf in early-summer, Hutchison said they may not even appear this year.
According to McVitie's Biscuit Market Report, Jaffa Cakes was the eighth-ranked biscuit brand by sales in the grocery sector, which includes supermarkets, in 2000.
In 2001, it had slipped to tenth, but last year, it regained the eighth spot, with sales of £19.9m.
This article was first published on Marketing