No other agencies pitched for the business, which was offered to Mason Williams after two months of discussions.
‘Weight Watchers is repositioning the way it talks to people and the technique of how it helps people will change,’ said Mason Williams MD Rita Rowe.
‘It is to make [the brand] more relevant to people today, to get people to understand how [they] get into an unhealthy lifestyle and how to change it permanently,’ she added.
The company said that its last PR agency, Barclay Stratton, worked with it more than six years ago.
The move coincides with the increasing popularity of the celebrity-endorsed Atkins diet.
Weight Watchers has become a household name since it started in the US in 1963 and launched in the UK in 1967.
It claims to have helped millions with its ‘eating regime’, which operates with a points system to enable dieters to eat whatever they like in controlled amounts.