By Ruth Bender And Stacy Meichtry 

PARIS--French conglomerate Bouygues SA Tuesday rebuffed an offer from Patrick Drahi for its telecom unit, a blow to the billionaire investor's efforts to become France's No 1 mobile operator and a victory for France's economy minister, who vehemently opposed the deal.

Bouygues said its board of directors unanimously decided not to pursue the proposal from Mr. Drahi's firm, Altice SA, which according to people familiar with the matter, offered to pay around EUR10 billion ($11.3 billion) for rival Bouygues Telecom.

The swift rebuttal defied investors who drove up telecom shares on expectations that Mr. Drahi's price tag--which valued Bouygues Telecom roughly twice that of most estimates--would be difficult for Bouygues Chairman and Chief Executive Martin Bouygues to reject.

Bouygues said its decision reflects the significant growth potential it sees in the French market. It said there was also a high risk the acquisition would have been rejected by regulators. The company said it was concerned too about the fallout from the proposed tie-up on jobs in the telecom sector.

The rejection sends a chilling signal to investors who have long questioned whether business leaders have a free hand in arranging disruptive deals in France. French Economy Minister Emmanuel Macron waged a highly public campaign against Mr. Drahi, describing his bid as a vehicle for "killing jobs."

Bouygues's board didn't bend to political pressure, a person familiar with the matter said Tuesday. For Mr. Bouygues, the heir to the founder of the conglomerate, the telecom business has almost sentimental value, people close to him have said.

Spokespeople for Altice and Mr. Macron declined to comment.

Mr. Macron, a former investment banker, has spent a large part of his first year in office seeking to lure investors back to France. He distanced the government from his predecessor, Arnaud Montebourg, who espoused dirigiste economic policies designed to exert government control over France Inc. On Wednesday, he is due to travel to the U.S. to renew his business-friendly pitch to American investors.

Mr. Macron's intervention in Bouygues, however, reveals how the minister is using his financial prowess to outmaneuver some of Europe's most cunning businessmen, from Mr. Drahi to Renault-Nissan Chief Executive Carlos Ghosn.

The economy minister was fast off the mark in publicly laying out his case against Mr. Drahi's proposal. The minister learned Mr. Drahi was homing in on Bouygues last week, according to a person familiar with the matter, before news of the proposal leaked to the French media. Mr. Macron then warned an Altice takeover of Bouygues risked creating a "too-big-to-fail" telecoms operator. He also took aim at Mr. Drahi's use of debt to fuel his acquisitions, raising the specter that taxpayers would have to foot the bill if the telecoms group ran into financial trouble.

On Tuesday evening, Mr. Macron met with the billionaire to talk about preserving jobs. Leaving the ministry, Mr. Drahi remarked: "The ball is now in the opposite camp."

A short while later, Bouygues announced it was shunning the bid, echoing Mr. Macron's concerns about potential job losses if the deal went through.

"The board took a close look at the consequences of consolidation on the labor market as well as the social risks necessarily tied to such an operation," Bouygues said.

For Mr. Drahi, the decision is a major setback in his quest to create France's largest telecom operator that would have allowed the group to create massive cost savings and assure higher profit growth.

Mr. Drahi last year bought SFR, France's Number 2 mobile company, from Vivendi SA for $23 billion. Since merging the mobile company with his cable company Numericable, Altice drastically cut costs, outraging the French government and worker unions.

Berenberg analysts estimated that Altice would have been able to create synergies of around EUR850 million a year in a Bouygues Telecom merger.

A merger would have helped all French operators shore up profits by easing competition, analysts say. French telecom companies have suffered sharp drops in revenue and profit since the arrival of low-cost rival Iliad SA on the mobile market three years ago sparked a bloody price war.

"We are convinced that the French telecom market can't support four operators in the long term," said a spokesman for state-owned Orange SA, France's largest telecom company.

Operators have been pushing hard to merge.

Besides Altice, Iliad has also made attempts to buy Bouygues Telecom in the past year and even Orange tried to find a way to consolidate but talks always collapsed. When Bouygues fought against Altice last year over SFR, the group argued that it would make sense to create a national champion to better compete in an Internet age.

Since then, Mr. Bouygues has played hardball. Asked in February if he would sell Bouygues Telecom, he replied: "Would you sell your wife?"

Mr. Bouygues began dreaming of entering the telecoms business back in the 1980s and later jumped on the opportunity to buy France's third mobile license, five years after being handed over the power from his father.

Bouygues in the past year has tried to get profits back on track by cutting jobs and launching cheaper broadband offers to win customers. The company said Tuesday profit margins should return to levels before Iliad's arrival in 2012, driven by consumers rapid shift to consumer data-heavy services via their mobile phones.

The risk of not getting a green light from regulators also weighed in its decision, Bouygues said, at a time when European regulators are divided over the benefits of telecom mergers.

Any deal would have face a review from France's competition authority that would likely have lasted eight or nine months due to the size of the proposed merger.

Write to Ruth Bender at Ruth.Bender@wsj.com and Stacy Meichtry at stacy.meichtry@wsj.com

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