The agency secured the business following a three-way shoot-out against Clark McKay & Walpole and Archibald Ingall Stretton. The AAR handled the review.
Kitcatt Nohr will work on all of Britannia's below-the-line corporate and consumer communications.
Future campaigns will promote Britannia's mortgage, savings and loans products and will include direct mail, digital, in-branch point-of-sale advertising, and posters.
The DM business was previously handled by Carlson, but the account was moved in- house last year.
The company's ad account, which is handled by McCann Erickson Manchester, is not affected. Media planning and buying will remain with Manning Gottlieb OMD.
McCann created Britannia's most recent TV ad "fair society", which featured an American talking about how fairness is represented in Britain.
Louise Fowler, the head of marketing at Britannia, said: "We felt that there was a natural fit between Kitcatt Nohr and Britannia."