The company began hunting for a new agency after experiencing a 15 per cent decline in membership in the past year. It is backing the initiative with a £7 million media spend.
Weight Watchers attributes the decline to the growth of fashionable diets and online health regimes, although some analysts have argued that the "Fat Fighters" sketch in Little Britain has stigmatised clubs such as Weight Watchers.
The appointment follows a competitive pitch against Isobel and Clemmow Hornby Inge, which already handles Weight Watchers' licensed food business. Both agencies have been informed that they were unsuccessful.
Neither Weight Watchers nor Euro RSCG London were available to comment.