Carat already handles Abbey's £26 million media planning and buying account for traditional media, which was shifted from MindShare in November.
The digital brief, which includes the Cahoot brand, covers all digital media and search activity. It was previously held by MindShare's digital division, mOne.
Last year, Carat picked up Abbey's £26 million media business after a four-way shoot-out against MindShare, OMD and Universal McCann.
The appointment marked a U-turn for the bank, which used Carat for more than a decade before it appointed MindShare in 2003. The decision to call a review was precipitated by the appointment of Luqman Arnold as the chairman of Abbey National.
Abbey was taken over by the Spanish financial services company Banco Santander in a £8 billion deal last year. The group is in the process of transforming the former building society from its historical focus on savings and mortgage-lending into a full retail bank.
Jeremy Davies, Abbey's director of brand and communications, said: "We have worked very well with mOne over the past few years and we'd like to thank mOne for their input in developing our digital communications strategy".