French telecoms operator Iliad has made a $15 billion cash offer to purchase 56.6 percent of T-Mobile USA, the No. 4 U.S. mobile provider, an unexpected move that counters an earlier merger proposal by Sprint.
T-Mobile confirmed to CNBC that it received a proposal from Iliad, but said it had no further comment on the matter at this time.
Iliad submitted the bid to T-Mobile's board of directors and said it is working with leading international banks to secure financing for the deal.
"The U.S. mobile market is large and attractive," Iliad said in a statement. "T-Mobile US has successfully established a disruptive position, which in many ways, is similar to the one Iliad has built in France."
The company said it values the remaining 43.4 percent of company at $40.5 per share on the basis of $10 billion of synergies to the benefit of the T-Mobile U.S. shareholders.
Dow Jones first reported the news, citing a an unnamed source who said Iliad views the bid as a "one-time opportunity to enter the world's-largest telecoms market."
Iliad has disrupted the French mobile and broadband market over the past few years with its ultra-cheap subscriber plans.
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Japan's SoftBank, which controls Sprint, and T-Mobile owner Deutsche Telekom AG had reportedly agreed to broad terms of a merger between the two firms, under which Sprint would pay around $40 per share for T-Mobile, valuing the smaller rival at nearly $32 billion, according to Reuters, who said the deal was expected to occur sometime after September.
S&P Capital IQ's Angelo Zino has a hold recommendation on T-Mobile and he believes Sprint can top the Iliad's offer, but says a merger of the two companies would endure intense scrutiny.
"We believe Sprint remains highly interested in acquiring [T-Mobile] and can provide a more superior offer to its shareholders," Zino said, adding that a potential offer from Sprint would likely face "significant regulatory hurdles as opposed to Iliad's."
Sprint shares dropped as much as 8 percent after the announcement, while T-Mobile shares rose more than 7 percent after being halted earlier.
"The opportunity for a transaction that brings capital and spectrum would be highly beneficial,'' T-Mobile CEO John Legere told CNBC earlier Thursday. "The opportunity to bring Sprint and T-Mobile together is one. But there are many different ways to to this and we will consider all of them."
During the second-quarter, T-Mobile added 1.5 million net customers in the and 102,000 net prepaid subscribers, compared with prepaid subscriber losses for the top four wireless carriers, according to Reuters.
Looking forward, the company raised its full year guidance and says it now expects to add between 3 million and 3.5 million postpaid customers in 2014.
—By CNBC's Karma Allen. Reuters contributed to this report.