Britain's competition watchdog on Monday launched a probe into the proposed merger between advertising giants Omnicom and Interpublic, to assess whether the deal could lead to a reduction in competition in the UK market.

Omnicom struck a $13.25 billion all-stock deal in December to buy rival Interpublic Group, creating the world's largest advertising agency. The deal is expected to close in the second half of the year.

The ad companies did not immediately respond to requests for comment.

The Competition and Markets Authority has set a deadline of August 13 for its phase 1 decision.

Reuters, citing sources, reported last week that the deal is also being reviewed by the U.S. Federal Trade Commission, which may require the combined company to agree not to boycott advertising on platforms based on political content.