Grey in New York has handled the Unilever-owned weight-loss product's advertising since 1987, but WPP Group's Ogilvy & Mather worked on the recent launch of brand extension Optima, which has been a success for the company.
It is believed that the agreement to move the account to O&M in New York was made before the acquisition of Grey Global Group by WPP was under way.
The US ad campaign from O&M is set to break in January 2005, with press ads breaking later this month. O&M will also handle public relations in the US.
Earlier this month, O&M also picked up work from Delta Air Lines to create a strategic brand campaign.
Slim-Fast has recently experienced increasing competition from the likes of fad diets such as Atkins and The South Beach Diet.
Sales of the brand, for which Unilever reportedly paid £1.4bn three years ago, have dropped by almost 30% since the Atkins Diet started to become hugely fashionable.
In the UK, it recently launched a summer campaign tempting British women to drop a bikini size by comparing them with "stick thin" French, Spanish and Swedish models.
The creative, which is ran in national press, women's weeklies and posters, includes images of thin continental models, with captions like, "I love British women... they make me look great" and "Face it British Women, it's not last year's bikini getting smaller".
Meanwhile in the US, comedienne and actress Whoopi Goldberg was dropped from a US ad campaign for the brand after making derogatory comments about President George W Bush at a Democrat fundraising event.
Goldberg, who starred in the films 'The Color Purple' and 'Ghost', made a joke at an event in New York, playing on the pun "Bush" to refer to a woman's genital area.
Terry Olson, general manager and vice president of marketing for Slim-Fast, said: "The O&M team delivered terrific creative work for the launch of our new Optima Diet plan and portfolio of products, and did their work in a fully integrated way across a range of channels."
The Optima Diet and foods are a shift in approach for Slim-Fast, cutting the levels of sugar by up to 55% in an attempt to appeal to the low-carb dieters.
According to Nielsen Monitor Plus, Slim-Fast spent $90m on ads and $35m through July 2004.
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