The agency won the account after a competitive pitch against Now and CHI & Partners. The win is expected to be ratified by the Co-op board next week.
DDB is believed to have withdrawn on the eve of the pitch as a result of its merger with agency Adam & Eve, which handles the John Lewis advertising account.
The review of the account, which began in January, was handled by the AAR. Pitches took place this week.
TBWA\Manchester was the incumbent on the main retail account and has been its lead creative agency since 2010, winning the account as part of its overall roster review.
Eleven of the Manchester-based Co-op's 14 roster agencies were based in the North-West. The Co-op's other interests, aside from food retailing, include financial services, funeral care and travel agents.
The Co-Op's media planning and buying account is handled by Rocket.
In January last year, the Co-op appointed the former MasterCard and John Lewis marketer Gill Barr as its group marketing director.
Leo Burnett was unavailable for comment.