By Ruth Bender 

PARIS--French telecom tycoon Xavier Niel has agreed to buy Orange Switzerland from Apax Partners in a deal worth EUR2.3 billion ($2.8 billion), as one of France's best-known entrepreneurs continues to expand his telecom assets across Europe.

Apax Partners said it has agreed to sell Orange Communications SA to NJJ Capital, Mr. Niel's private holding company, roughly two years after the private-equity fund took control of the Swiss mobile operator.

The move highlights Mr. Niel's growing interest in expanding his telecom portfolio outside of France. Earlier this year, the founder of French telecom firm Iliad SA also bought Monaco Telecom after a failed attempt to buy U.S. mobile company T-Mobile US Inc. from its German parent Deutsche Telekom AG.

The French businessman managed to finally get his hands on Orange Switzerland, having failed in a final bidding round to buy the company in 2011, but at a higher price. Back in 2012, Orange SA--formerly known as France Telecom SA--sold Orange Switzerland to Apax Partners for EUR1.6 billion.

Mr. Niel said he has followed Orange Switzerland's developments very closely since.

"NJJ Capital is a long-term strategic investor, owned and operated by telecom professionals with proven record whose goal will be to share and exchange best practices with Orange Switzerland management," Mr. Niel said in a statement.

The deal is one of the largest Mr. Niel has done as a personal investment. The programmer-turned-billionaire has mostly done acquisitions under the $1 billion mark, investing mainly in startups, where he has had considerable success.

In France, Mr. Niel is known as the "troublemaker" within the telecom scene, having sparked a price war among mobile operators when his firm Iliad SA entered the mobile market in 2012. He also owns a large stake in Israeli telecom company Golan Telecom, which has followed a similar strategy to Iliad in France by keeping costs at a minimum and offering ultracheap tariff plans.

Analysts have been speculating what Mr. Niel next move might be after he failed to buy T-Mobile US.

People familiar with the matter have said that while Mr. Niel would likely continue to buy small to medium-size telecom companies in the future, Iliad will eventually want to go for another big, transformative deal.

The deal for Orange Switzerland is expected to close by the end of the first quarter, 2015, Apax said.

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

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