The network already works with the brand in the US and the move is part of a global alignment. It secured the business in April 2007, following a pitch against DDB, BBDO and Deutsch.
The business will cover the UK, as well as key European markets. It will be run out of the London office. The brand spent £8 million on UK media last year, according to Nielsen.
Euro RSCG, the UK incumbent on the account, won the business in September 2006, after a pitch against Isobel and CHI & Partners.
This is the first time that Weight Watchers has used one agency across Europe and it is expected that McCann will draw up a unified advertising strategy. This is likely to cover the creation of a campaign based around retaining existing members, while increasing subscriptions by making the offering more relevant to today's dieters.
A similar concept has been implemented by McCann in the US, where the agency came up with the strapline "Stop dieting, start living" in order to promote a sensible approach to dieting.