The news comes as a blow to the incumbent, DDB London, 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 past few weeks. WCRS was awarded the account, which includes the Ready Brek, Weetos and Alpen brands, this week.
WCRS came close to winning the Weetabix account in the review a year ago. Lowe was also a runner-up 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's 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.