(Reuters) - SoftBank Group Corp CEO Masayoshi Son faces a closing window of opportunity to merge the Japanese company’s debt-laden U.S. wireless subsidiary Sprint Corp with Deutsche Telekom’s U.S. wireless unit, T-Mobile U.S. Inc.

Smartphones with the logos of T-Mobile and Sprint are seen in this illustration taken September 19, 2017. REUTERS/Dado Ruvic/Illustration

Sprint’s latest round of merger talks with T-Mobile, reported earlier this week, came just five months after their previous deal negotiations ended. This is the companies’ third attempt to clinch a deal in the last four years.

While the time window for a deal is not clearly defined, the companies need to act before a U.S. wireless spectrum auction in November. Sprint’s and SoftBank’s deteriorating finances and the 2020 U.S. presidential election also make it unlikely they will get another chance, sources close to the negotiations and analysts said.

“The view is, if they are going to pursue a merger, now is the time,” Macquarie analyst Amy Yong said. “T-Mobile can help them turn around the network.”

SoftBank controls Sprint through its 85 percent stake. Son pulled out of merger talks with T-Mobile in November after he changed his mind on the valuation he would accept. His willingness to entertain a deal again led to the negotiations restarting, sources have said.

Sprint is saddled with long-term debt of more than $32 billion and has also signaled that its capital investment is going to double, as it seeks to build out its network to compete against T-Mobile, Verizon Communications Inc and AT&T Inc. Analysts have warned Sprint will struggle to handle this burden on its own unless it cuts a deal with T-Mobile soon.

“With a significant step-up in their network capex of an incremental $1.5 billion to $2.5 billion year-on-year beginning in fiscal 2018, it is hard to see Sprint coming close to free cash flow breakeven, much less cash flow positive, in the foreseeable future,” MoffettNathanson LLC analyst Craig Moffett wrote in a note.

FILE PHOTO: SoftBank Group Corp Chairman and CEO Masayoshi Son speaks at SoftBank World 2017 conference in Tokyo, Japan, July 20, 2017. REUTERS/Issei Kato

A Sprint spokeswoman declined to comment on whether the company was under time pressure to complete a merger with T-Mobile. T-Mobile and SoftBank did not immediately respond to requests for comment.

SoftBank has its own mounting financial constraints following a string of acquisitions and needs to pay down debt, which reached 15.8 trillion yen ($147 billion) as of the end of December. It has said it is planning to raise cash by taking its Japanese mobile phone unit public this year.

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