The soft drinks giant has started talks with agencies, including UK shops, ahead of a contest for a place on the roster.
Mother, the incumbent on the £4m Diet Coke account in the UK, will no longer work on the brand, but will continue to handle global and UK Olympic campaigns for Coca-Cola.
The Diet Coke agency process follows the announcement by Jonathan Mildenhall, the Coca-Cola vice president of global advertising strategy and creative excellence, at the Cannes International Festival of Creativity that the company is moving towards an open model, which he described as "liquid and linked".
A spokesperson for Coca-Cola said: "We are looking for a new way of working on the brand in North-West Europe as we take its collaboration with fashion to the next stage.
"We are therefore exploring partnering with a combination of content agencies with different expertise for 2012 and beyond."
Mother became the agency of record for Diet Coke in August 2008 after a pitch. Before that, VCCP handled the business.
Diet Coke's recent TV advertising campaign centred around puppets styled as fashionistas and used the strapline, "love it light".
The most recent TV work in the campaign, called "handbag", featured the puppets in a store cutting up an oversized handbag with a chainsaw.
Schweppes, the Coca-Cola owned soft drinks and mixer brand, is currently holding a pitch for a new campaign.