The move dents the ambitions of WPP's Red Cell network to repeat the success it has had in gaining a foothold on Coca-Cola's core brand advertising account in the US.
Red Cell acquired the New York-based agency Berlin Cameron in December 2001 and now shares the Coke account with McCann in the US.
A month later, WPP approached and was rebuffed by Mother to acquire it to form the basis of the Red Cell network in the UK to mirror the US move.
Mother, which handles advertising for Coke brands including Lilt and Dr Pepper, has now been called in by Coca-Cola to look at the flagship brand. Coca-Cola denies the move threatens McCann.
"McCann-Erickson is our agency of record. There is no pitch process in place, everything else is speculation," a spokeswoman for Coca-Cola (GB) said. "The other point is that for a good few years the philosophy is to think local and act local."
Andy Medd, a partner at Mother, commented: "We're not pitching for Coke. We work on a lot of strategic projects across a wide range outside of straight advertising."
The brand is backed by a budget in excess of £15 million above the line in the UK. Recent local work by McCann has included its "Leggsy" animated work to support its World Cup sponsorship. However, the brand continually recycles old campaigns, including its "holidays are coming" spot for Christmas.
Under its marketing director, Charlotte Oades, Coca-Cola (GB) has shifted a large tranche of its budget into small, local agencies. Mother handles Schweppes, Oasis, Lilt and Dr Pepper, Soul has Fanta, and Lowe does Sprite and Diet Coke. Coca-Cola has global deals with both IPG and WPP, however.