Britannia instructed regular adviser Allen & Overy, with corporate partner Richard Slynn advising on the merger.
Co-op shunned its panel firms and turned to Slaughter and May corporate partner Simon Phillips. The group has previously instructed Addleshaw Goddard and Clifford Chance for large scale corporate transactions, while Eversheds, Hammonds, Herbert Smith and Watson Burton are also on its joint panel.
The merger is dependent on the passage of new legislation allowing the different types of mutual to combine.
Slynn said: “We worked closely with the Treasury and the FSA to satisfy both that this could be done in accordance with the legislation.”
The combined group will have more than nine million customers. Britannia members will become members of the Co-op Group.
Co-op has used a range of corporate advisers in recent years. Addleshaw Goddard won the mandate for group’s £1.57bn takeover of Somerfield in July last year (21 July 2008), while Clifford Chance got the call for the company’s merger with United Co-operatives in August 2007 (6 August 2008).