PARIS--France's competition authority on Thursday raided several offices of mobile and broadband operator SFR-Numericable over concerns that the company might have breached merger rules, according to people familiar with the matter.

France's Autorité de la Concurrence searched the offices after rival Bouygues Telecom raised concerns with the competition authority over whether Numericable and SFR began working together on commercial offers before they received the official green light from the antitrust body, the people familiar with the matter said.

The competition authority confirmed that it was conducting a raid "in the telecommunications sector," but declined to comment further.

The raid comes a few months after mobile company SFR and cable operator Numericable officially merged following a takeover battle between Numericable's owner, cable tycoon Patrick Drahi, and the owner of Bouygues Telecom, Bouyuges SA .

Mr. Drahi won the takeover fight and paid EUR17 billion ($18.3 billion) to buy SFR, a move that left France with four mobile operators in an intensely competitive market. French telecom companies have suffered sales declines since the arrival of a fourth mobile company, Iliad SA, three years ago. Bouygues Telecom, which had suffered particularly hard in the price war sparked by Iliad's ultracheap offers, wanted to merge with SFR to cut the number of mobile firms in France back to three.

Bouygues Telecom in November last year addressed a letter to the competition authority stating that it believed Mr. Drahi already controlled decisions at SFR before the competition authority approved the merger in late November, the people said. In the letter, Bouygues Telecom said it believed teams from the two companies might have worked on a new set-top box ahead of the merger approval, the people added.

SFR-Numericable on Thursday informed its employees that officials from the antitrust watchdog have sealed off certain offices as part of an investigation, according to an email to staff reviewed by The Wall Street Journal.

A dozen officers from the antitrust body were searching through documents and computers at the group's headquarters north east of Paris and in Numericable offices in a suburb east of the capital, one of the people familiar with the matter said.

Write to Ruth Bender at Ruth.Bender@wsj.com

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