The decision sees Ogilvy & Mather, the incumbent on the business, removed from Lucozade's roster in the UK. However, the agency will continue to run the business in Ireland.
GSK put the account up for pitch in November 2005 as it reconsidered its marketing strategy for the brand.
Both roster agencies were asked to pitch against each other for the business.
No other GSK roster agencies were involved.
With the appointment of M&C Saatchi, the pharmaceutical giant is looking to rejuvenate the Lucozade brand by bringing all of the business under one roof and developing a single overarching idea.
M&C Saatchi now handles all of Lucozade's £9 million drinks business, including the £4 million Hydro Active and Sport brands.
It picked up the latter in 2003 following a three-way pitch against O&M and TBWA\London, which was not on the agency roster.
GSK then handed the Hydro Active business to M&C Saatchi without a pitch and gave the agency the task of launching the brand.
In February this year, Lucozade appointed Fuse Digital to handle all of its digital activity, including website creation and e-mail marketing.
The agency is developing websites for all of the Lucozade products, as well as for the Lucozade Sports Science Academy.