UPDATE 2-Chairman of U.S. telecoms regulator backs T-Mobile, Sprint merger

By David Shepardson and Diane Bartz

(Adds details, background, stock prices)

By David Shepardson and Diane Bartz

WASHINGTON, May 20 (Reuters) - Sprint Corp and T-Mobile US Inc has won the support of the chairman of the U.S. telecommunications regulator but will require a series of changes to their proposed $26 billion merger, including selling Sprint's Boost Mobile cell service.

Federal Communications Commission (FCC) Chairman Ajit Pai said on Monday he will recommend that the other four commissioners vote to approve the merger. A second member of the five-person FCC, Commissioner Brendan Carr, a Republican, also said he will vote to approve the deal.

Once an order is drafted, the full panel must vote to approve or reject the deal.

Sprint jumped 25.1% to $7.73 while T-Mobile was up 6% to $79.93.

The FCC will not formally vote on the merger on Monday but will first draft an order, two people briefed on the matter said. The Justice Department must also approve the deal.

In a filing with the FCC on Monday, the companies pledged to sell Boost Mobile, a prepaid wireless provider.

Altice USA, the fourth largest cable company, has urged the FCC to reject the deal because it was concerned the combined company could prevent it from offering phone service.

Altice has a Mobile Virtual Network Operator agreement with Sprint that would allow it to start offering mobile phone service later this year. The companies pledged they would not scrap the deal with Altice.

T-Mobile also promised that the new company would build a "world-leading 5G" network, the next generation of wireless service, to give rural Americans robust 5G broadband and enhance home broadband.

T-Mobile had said the companies were extending a deadline to complete their deal to July 29. The two are among just four national wireless carriers, with Verizon Communications and AT&T Inc leading the pack.

The FCC and Justice Department had been expected to make a decision in early June. They have been weighing a potential loss of competition, and subsequently higher prices for consumers, against the prospect of a more powerful No. 3 wireless carrier that can build a faster, better 5G network. (Reporting by David Shepardson and Diane Bartz; Editing by Susan Heavey, Paul Simao and Jeffrey Benkoe)