T-Mobile US Inc agreed on Sunday to buy Sprint Corp, in an all-stock deal for $26 billion that will combine the third and fourth largest U.S. wireless carriers.

The agreement caps four years of talks between the companies, snd will result to the creation of a carrier with 127 million customers that will be a more formidable competitor to the No.1 and No.2 wireless players, Verizon Communications Inc and AT&T Inc.

U.S. regulators, which have challenged in court AT&T's $85 billion deal to buy U.S. media company Time Warner Inc, are expected to probe Sprint and T-Mobile on how they will price their combined wireless offerings.

The breakthrough in the companies' negotiations came after T-Mobile majority-owner Deutsche Telekom AG and Japan's SoftBank Group Corp, which controls Sprint, agreed on a structure that will allow Deutsche Telekom to continue to consolidate the combined company, which will have a market value of over $80 billion, on its books.

Deutsche Telekom will own 42 percent of the combined company, and will control the board of the combined company, nominating nine of the 14 directors.

Following closing, the new company will be headquartered in Bellevue, Wash., with a second headquarters in Overland Park, Kan. John Legere, current President and Chief Executive Officer of T-Mobile US and the creator of T-Mobile's Un-carrier strategy, will serve as Chief Executive Officer, and Mike Sievert, current Chief Operating Officer of T-Mobile, will serve as President and Chief Operating Officer of the combined company. The remaining members of the new management team will be selected from both companies during the closing period. Tim Hottges, current T-Mobile US Chairman of the Board, will serve as Chairman of the Board for the new company. Masayoshi Son, current SoftBank Group Chairman and CEO, and Marcelo Claure, current Chief Executive Officer of Sprint, will serve on the board of the new company.

The new company will have some of the most iconic brands in wireless - T-Mobile, Sprint, MetroPCS, Boost Mobile, Virgin Mobile - and will determine brand strategy after the transaction closes.

Sprint's and T-Mobile's first round of merger talks ended unsuccessfully in 2014 after U.S. President Barack Obama's administration expressed antitrust concerns about the deal.

Both Sprint and T-Mobile are behind Verizon and AT&T in upgrading their network to accommodate next generation 5G wireless technology. Even after their merger, the combined company's budget to invest in 5G will be smaller than Verizon or AT&T's.

'However, Sprint and T-Mobile hope the deal will give them more firepower to participate in auction for spectrum to develop 5G.

The two companies expect the transaction to close no later than the first half of 2019.