Rivals question $35 billion global ad deal
"In terms of size, at the end of the day, if you look at the geographies, it doesn't really create great scale except in the U.S. and obviously the regulators are going to take a good hard look at that," Sorrell told CNBC Europe's "Squawk Box."

The global advertising market began its week with news of the creation of the world's biggest firm in the sector, after France's Publicis and U.S.-based Omnicom announced at the weekend they were joining forces to form a group worth $35.1 billion, overtaking global leader WPP.

David Jones, chief executive of French advertising agency Havas, said he was "very surprised" by the merger, and questioned the rationale of the deal for clients and employees.

"It's clearly a very cleverly constructed deal and I think it makes huge sense for the two current CEOs and works for them, but I think you end up making two people happy and 130,000 [people] and many clients concerned and destabilized," he told CNBC's "Worldwide Exchange."



Jones said Havas wouldn't block the deal for anti-trust reasons.



Both WPP's Sorrell and Havas's Jones said a bigger agency wasn't what clients wanted, rather marketers wanted agencies that were more nimble, entrepreneurial and less bureaucratic.

Shares of WPP closed 0.6 percent higher on Monday, while Havas's shares were up 4.68 percent. Shares of Publicis were over 3.7 percent higher at 5.15 p.m after the London market closed.



Analysts also questioned whether Omnicom and Publicis could execute on the merger.

Potential issues include how the firm will cope with its bases split between the U.S. and Europe, whether the two cultures can be combined effectively and potential conflicts of interest in terms of representing competing clients.

The firms will now collectively represent competing brands, including Coca-Cola and Pepsi, McDonald's and Taco Bell.



"Clients really have not been taken through the pros and cons of this in any great detail and it's going to be very interesting to see what happens," WPP's Sorrell said.

Mike Amour, CEO of Asia Pacific at advertising network Project Worldwide, said the merged firm could tackle this concern by being open and transparent with its clients.



"As long as the agencies are transparent and are upfront with their clients about potential conflict [the issue can be avoided]. This is not the first time this has happened in regards to clients from similar categories sitting under the same agency," he said.

(Read more: Publicis CEO: Digital, US rebound driving revenues)



Publicis's 71-year old CEO Maurice Levy and Omnicom's 60-year old CEO John Wren will run the merged group together for the first two-and-a-half years, after which Wren will take the reins.