Premier Foods has been responsible for producing and promoting the low-calorie hot-drink brand for 20 years, but Cadbury is keen to oversee the brand itself.
It plans to move Highlights into additional sectors and has already extended the range to include teacakes, to compete with brands such as Weight Watchers.
BBH had been responsible for creating ads for Highlights and also holds the contract for Cadbury's biscuit brand range, which is manufactured and promoted by Burton's Biscuits. The bulk of the Cadbury chocolate business is held by Publicis.
A spokesman for Cadbury said it was too early to speculate on the marketing implications for the brand.