Havas Media has looked after Britain’s biggest building society since 2008 and pitched against Starcom and the7stars as well as WPP’s MEC.
Nationwide kicked off the media review earlier this year within weeks of Sara Bennison, formerly of Barclays, joining as chief marketing officer and Joe Garner, previously head of BT Openreach, starting as chief executive.
A Nationwide spokesman declined to confirm any change, saying: "Nationwide is currently reviewing its media planning and buying agency and, although this is in its final stages, we are unable to provide any additional information at this time.
"We will make a further announcement once the process has been fully completed and all parties involved have been informed."
MEC declined to comment.
Nationwide warned at last week’s quarterly results that profits could take a hit because of subdued economic conditions following the Brexit vote but promised to focus on customer "loyalty and advocacy".
The company said: "The sustained low interest rate environment and competition in core markets will maintain pressure on margins and we anticipate profits are likely to moderate in the period ahead."
The building society spent £37.8m on media in the year to June, including £22m on TV, based on Nielsen data.
Bennison was chief marketing officer at Barclays when it appointed another WPP media agency, Maxus, to handle media.