The new business was legally formed 1 August after members voted in favour but the two will continue to develop separate marketing campaigns.
A spokesman for the Co-operative says the intention is to “retain the identity of both” though separate activity.
However, the two will look to offer consistent rates across similar products “as soon as possible”.
Both Co-operative Bank and Britannia branches will also be retained, with the latter accounting for 210 of the combined businesses 300 branches.
In a statement, Neville Richardson, chief executive of The Co-operative Financial Services, says the official launch of what has previously been described as a “super mutual” is the “next step in the renaissance of the co-operative and mutual sector.”
“Trust has become a scarce commodity in other financial businesses of late and we aim to provide a genuine alternative to those disillusioned with shareholder owned banks,” he says.
Co-operative Financial Services, part of the Co-operative Group, includes Co-operative Insurance, Co-operative Investments and the Co-operative Bank.
It currently uses a number of creative agencies including Farm for savings and Rapp Edinburgh for insurance while PHD Rocket handles media. McCann Erickson runs Britannia’s advertising account and Manning Gottlieb its media.
The Co-operative group, which also includes supermarket and travel businesses, is currently reviewing its multimillion pound account.