The review coincides with plans by Weight Watchers to emulate its US strategy by opening high-street stores in the UK in a bid to modernise its brand.
The company traditionally divides its advertising between its range of food and drinks, some of which are licensed to Heinz and are not affected by the review, and its slimming club.
It has already contacted agencies to invite them to pitch for the business.
Isobel works on the Weight Watchers food range and McCann Erickson oversees advertising for the brand's slimming club.
VCCP handles the Weight Watchers/Heinz joint venture, which does not constitute part of the pitch.