The agency, which had held the business until it moved to MindShare two years ago, triumphed in a pitch against MindShare, Universal McCann and OMD UK.
The final stage of the pitch process took place on Friday when the remaining agencies submitted pricing proposals online. Carat will work for Abbey and its online brand Cahoot.
Carat's appointment follows a series of changes at Abbey since its acquisition by Grupo Santander last November. It recently appointed WCRS as its ad agency, replacing TBWA\London.
Jeremy Davies, the director of brand and communications at Abbey, said: "The review was aimed at getting better value by bringing together the spend of Abbey and Cahoot.
"We've undergone major changes to our brand and advertising since the beginning of this year and now we need a fresh approach to getting the best value from our media planning and buying. We look forward to working with Carat to support these changes."
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