The decision sees both firms retain their regular corporate instructions from the insurance giants - Clifford Chance for CGU and Slaughters for Norwich Union.
Norwich Union says its current panel review, which includes an investigation into possible overcharging for bulk litigation work, had no effect on its choice of Slaughters for the deal.
"There is a review under way but for a merger of this size there is really only very few firms who can handle it," says a Norwich Union spokes-man. "We saw no reason not to stick with Slaughters."
The £19bn merger will create the UK's biggest provider of general insurance.
Slaughters' legal team will be headed Glen James, with corporate partner Adam Signy taking on the same role for Clifford Chance.
Slaughters corporate partner Jonathan Marks says the firm has a team of 15 lawyers to work full-time on the merger, including corporate partner Tim Clark, tax partner Howard Nowlan, EC competition partners Malcolm Nicholson and William Sibree and pensions/employment partner Jonathan Fenn.
"We expect to achieve completion by early June. There are many regulatory hurdles to overcome before we can proceed with the merger," he says.
A spokesman for CGU says it has not considered any changes to its panel following the merger announcement.
The company is known to use Lovells for corporate litigation, Kennedys for insurance and Masons for IT. CGU cut its panel from 100 firms to 18 in August last year, after Commercial Union merged with General Accident.