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The United Kingdom’s Competition and Markets Authority (CMA) has announced an initial review of the proposed $13 billion merger between U.S.-based marketing giants Omnicom Group Inc. and Interpublic Group of Companies Inc. This preliminary step marks the beginning of the UK’s scrutiny of the high-profile transaction, already under regulatory examination in the United States.

The CMA issued a public invitation to comment on whether the merger may harm competition in any UK markets. The invitation is open until May 21, allowing interested stakeholders to submit their views.

“This invitation to comment is the first part of the CMA's information-gathering process,” the regulator stated. The CMA clarified that this does not yet constitute a formal investigation.

The Omnicom Interpublic $13B merger was announced in December as a stock-for-stock deal, with Omnicom offering 0.344 shares for each Interpublic share. Upon closing, Omnicom shareholders will hold 60.6% of the combined company, while Interpublic shareholders will own 39.4%. The transaction, guided by Latham & Watkins LLP (Omnicom) and Willkie Farr & Gallagher LLP (Interpublic), is expected to generate $750 million in annual cost synergies.

In the U.S., the Federal Trade Commission (FTC) launched an in-depth investigation into the merger in March. Both Omnicom and Interpublic have described this as a routine part of the regulatory process and maintain that the deal is on track to close in the second half of 2025, pending all required approvals.

The merger has also drawn attention from U.S. lawmakers. In a letter to the FTC and Department of Justice, five Senate Democrats raised concerns over potential political interference, alleging that Elon Musk may be pressuring Interpublic through advertising-related threats linked to his social media platform, X. The letter suggested that such influence could impact the regulatory path of the merger under the Trump administration.

Representatives for Omnicom and Interpublic have not issued comments in response to the letter.