Abbey has removed TBWA\ London from its £22 million account and replaced it with WCRS after a pitch called on 18 February.
TBWA repitched for the business against WCRS and Lowe. Leo Burnett was shortlisted but withdrew last week because of conflict with its Scottish Widows account.
The review followed the bank's takeover by Spain's Grupo Santander at the end of last year. It will see an end to the "Me and My Money" advertising strategy that was developed by TBWA. WCRS is tasked to start working on a new campaign, which will break in May.
The news comes as a further setback for TBWA, which experienced a difficult 2004, slipping from seventh to 15th place in the Nielsen billings rankings.
Earlier this month, it also lost its £20 million Sun and News of the World accounts. News Group Newspapers, the owner of the tabloids, is currently using the AAR in its search for a new advertising agency.
Jeremy Davies, the director of brand and communications at Abbey, said: "While we will be sorry to end our relationship with TBWA, which has proven to be a valuable and enthusiastic partner for the past two years, this is an appropriate time to make a step change in our advertising."
He added: "WCRS demonstrated a superior understanding of the requirements of our business and our customers, coupled with strong creativity."
WCRS has moved quickly to replace its loss of the £5 million First Direct business last year following the global consolidation of HSBC into WPP.
Abbey will sit alongside WCRS's existing financial services client, Prudential.
Debbie Klein, a founding partner of WCRS, said: "We are delighted to celebrate our first birthday as an independent agency by being chosen to work with Abbey."
Abbey will continue to work with Tequila on its below-the-line activity and MindShare on media planning and buying.
TBWA was awarded the Abbey account in 2003 when its chief executive, Luqman Arnold, reviewed Abbey's entire communications roster, handing TBWA the above-the-line business without a full pitch.