Blue 449 is believed to have snatched the account from OMD without a formal pitch.
Weight Watchers has used Blue 449’s sister agency Saatchi & Saatchi for its advertising since 2011.
Saatchi & Saatchi is understood to have introduced Blue 449 to Weight Watchers.
This is the second time this year the two shops have worked together to win business for Publicis Groupe. Saatchi & Saatchi and Blue 449 teamed up to capture Asda’s £90m-plus combined account from VCCP and Carat in April without a review.
Industry sources suggested Weight Watchers moved the business to Blue 449 because the brand wants more joined-up thinking from its agencies.
Audrey O’Brien, Weight Watchers UK marketing director said: "Saatchi & Saatchi manage our creative business and the decision to appoint Blue 449 is part of a drive to create more cohesive, integrated and efficient communications across an increasingly fragmented landscape."
She said OMD had been "a great partner" for the last nine years "but moving forward the integrated solution between Saatchi & Saatchi and Blue 449 is compelling and we’re looking forward to working with the new joint team".
Phil Georgiadis, chairman of Blue 449 UK, said: "Blue 449 are delighted to be working alongside our creative partners at Saatchi & Saaatchi on the Weight Watchers business. We are working hard to create a seamless creative and media model to drive maximum commercial effect.’’