Imagination, which is understood to be close to signing contracts with Co-operative, picked up the business after a closely fought final round against Mediaedge:cia.
The agencies were asked to respond to an open brief based on determining the future of Co-op Financial's image.
The company, which includes Co-operative Bank and Co-operative Insurance Society, markets itself on the basis of its ethical standards.
One source told Media Week that the project might be partly sponsorship-led and could lead to Co-op Financial reviewing some of its existing sponsorship deals.
The financial services company currently endorses second-tier rugby league and the Netball Superleague.
The pitch process was overseen by the Co-op's head of brand management, Kelvin Collins. A spokesman for the company said: "We have been in talks with two events companies, one of which is Imagination, but have not officially made an appointment."
Co-operative Group, which owns Co-operative Financial Services and businesses spanning supermarkets, funeral services and travel, uses PHD Rocket for its media buying. The agency is unaffected by Imagination's appointment and was not believed to have been involved in the pitch.