The news comes as a blow to incumbent DDB, which won the cereal account just over a year ago.
DDB and WCRS are understood to have been working head-to-head on a brief for Weetabix over the last few weeks.
WCRS came close to winning the Weetabix account during the last review. Lowe was also runner-up in the pitch and is thought to have been a contender to pick up the account this time round.
The decision to appoint WCRS was made by Weetabix chief executive Ken Wood and marketing controller Tony Corp.
Wood was responsible for appointing DDB London to the account in December 2004. It moved from FCB, which handled the business for two years, having won it from Lowe in 2001.
DDB's work for the brand has included the "scarecrow" TV spot, which featured a scarecrow who is nursed back to health by a farmer who feeds him wheat.
Weetabix's brands include Minis, Alpen, Ready Brek and Weetos. The company is also planning to launch a new muesli-type cereal under the name Oatabix.
No one at Weetabix or WCRS was available for comment at the time this story went to press.
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