The review aims to consolidate the business into one agency and follows the merger of Mars and Wrigley, which took effect in October 2008.
The UK review comes in the wake of Mars-Wrigley's appointment of the Publicis Groupe-owned MediaVest to its $550 million US media planning and buying account in May.
ZenithOptimedia handles media buying across Mars' confectionery, petcare and food ranges with brands including Snickers, Pedigree and Uncle Ben's. Spend across the business is close to £55 million.
The £8 million Wrigley media buying account, which is handled by MediaCom, includes brands such as Juicyfruit and Airwaves.
Planning duties are not expected to form part of the review. In February, ZenithOptimedia landed the £15 million planning assignment for Mars Petcare, breaking MediaCom's hold on the consolidated Mars planning account. MediaCom continues to handle planning across Mars food and confectionery brands in addition to Wrigley.