The switch means that McCann will add the £4 million creative account to the below-the-line business it handles for the society.
Alan Long, Britannia's head of marketing, said: "In addition to bringing a new creative direction, this move will allow us to fully integrate our communications. We will achieve this at every level through advertising, direct mail, online and at branches."
McCann, currently working on a new creative strategy due to break this spring on TV and in national newspapers, is also developing new brand guidelines for all Britannia's communications activity.
The move sees the account return to Manchester almost three years after Walsh Trott won the business from BDH/ TBWA in a four-way pitch.
Sue Little, the McCann chief executive, said: "One of the greatest challenges for advertisers within the financial sector is to produce communication that isn't only effective and relevant but also stands out from the crowd. We're really looking forward to doing exactly that for Britannia."