BRUSSELS--Advertising giants Publicis Groupe SA (PUB.FR) and Omnicom Group Inc. (OMC) were given the green light by European Union regulators Thursday for their $35.1 billion merger, paving the way for the creation of the world's biggest advertising agency.

The European Commission said the deal didn't pose any threats to competition and granted its approval without conditions.

The tie-up will see some of the world's biggest brands come together under one roof and will be known as the Publicis Omnicom Group.

The U.S.'s Omnicom is the world's largest ad agency and French Publicis is ranked third, behind WPP Group PLC (WPP.LN).

"The bidding nature of the markets, the presence of other large competitors, the relatively low barriers to entry, and the significant countervailing power of media vendors will ensure a level playing field in all the affected markets after the merger," the Commission said in a statement.

Omnicom's clients include fast food giant McDonald's Corp. and beverage multinational PepsiCo Inc., while Publicis has beauty house L'Oreal SA, consumer product firm Procter & Gamble Co. and automaker Renault SA on its books.

The new conglomerate will boast a combined revenue of nearly $23 billion, based on 2012 figures for each, overtaking main rival WPP.

Publicis also owns own the media agencies Saatchi & Saatchi and Leo Burnett, and Omnicom has BBDO Worldwide Inc. and DDB Worldwide Inc.

EU regulators said customers would have the choice to switch to rival agencies if the merged entity "should increase its prices or decrease the quality of its services."

--Suzanne Vranica and Ruth Bender contributed to this story.

Write to Vanessa Mock at vanessa.mock@wsj.com

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