The fitness equipment company is famed for its toning belts that use electrical stimulation to define muscle.
Cake's brief is to develop a PR campaign to launch a new product for the high-end beauty market this autumn.
The agency will work to reposition the brand by targeting the upmarket lifestyle and beauty media such as Vogue and Tatler. The campaign will include product placement, celebrity seeding and the creation of digital content.
Cake is also charged with increasing the profile of Slendertone more generally and promoting its existing products.