The brief combines the media duties for Abbey with those of Alliance & Leicester and the savings arm of Bradford & Bingley, both acquired by Santander in the past six months.
Publicis Groupe's Starcom failed to make the final shortlist while Omnicom's OMD withdrew from the running in November.
Mediacom North, the regional agency which previously handled media for Bradford & Bingley, was not invited to pitch.
The decision follows a pitch process that included a closed online auction on Wednesday (17 December), led by Abbey's brand and communications director Keith Moor.
Earlier this month, Moor's role at the Spanish financial group was expanded to include marketing across all three brands.
Carat originally won the Abbey media account in November 2005. Based on Nielsen figures for the 12 months to October 2008, the combined accounts were worth £48m, but the newly consolidated business only includes B&B's retail marketing and is expected to be worth closer to £38m in 2009.
Yesterday, EU competition authorities approved Santander's acquisition of Bradford & Bingley's savings arm.