The business is currently held by Mediaedge:cia, which has handled the account for the past 12 years.
The review will be handled by Oystercatchers, the agency search business that was recently launched by Suki Thompson, a founder of the Haystack Group, and Peter Cowie, the chief executive of FitchLive and a former head of new business at JWT.
Oystercatchers has sent out an initial request for information to agencies ahead of a competitive pitch, which will take place early next year. MEC has been invited to repitch for the business. The review does not affect creative on the account, which is handled by Leagas Delaney.
Peter Gandolfi, the head of brand strategy at Nationwide, said: "We have a long-standing relationship with MEC, which we have valued enormously.
"However, clearly, both the media and financial businesses have evolved hugely over the years, and it makes sense to review our systems and processes periodically."