Publicis CEO Maurice Levy said the reaction to the merger from all the company's major clients had been "extremely positive".

Competing sector executives and analysts expect that the Publicis Omnicom merger could spur some big customers to switch agencies to avoid conflicts.

Holding companies such as Omnicom and Publicis own networks of agencies and PR firms within the larger group. They're designed to work independently so that competing clients - Pepsi and Coca-Cola for example - are placed in separate networks to avoid conflict.

"We're going to work extremely hard to resolve any client conflict issues with creative solutions," said John Wren, the Omnicom CEO.

(Reporting by James Regan; Editing by Leila Abboud)

Stocks treated in this article : PUBLICIS GROUPE, Omnicom Group Inc.