It's official: T-Mobile and Sprint have announced plans to merge, with a combined company valued at $146 billion.

John Legere, T-Mobile's CEO, is expected to serve in that role for the merged entity, which would retain the T-Mobile name.

The agreement marks the culmination of four years of on-again, off-again discussions.

The deal is likely to draw scrutiny from antitrust regulators, considering the Trump administration's treatment of AT&T and Time Warner's attempted merger.

Sprint dropped by 13% in premarket trading on Monday, while T-Mobile slid 2.4%. Follow live trading on Markets Insider.

The boards of T-Mobile and Sprint have put the finishing touches on a massive merger agreement that values a combined company at $146 billion.

T-Mobile's CEO, John Legere, made the announcement on Sunday by tweeting a seven-minute video breaking down the merger and linking to a website that further explains the combination.

Deutsche Telekom, which owns two-thirds of T-Mobile, would control the newly formed company.

Legere is expected to be the CEO of the combined entity, which would keep the T-Mobile name and have headquarters in Bellevue, Washington, and Overland Park, Kansas.

The deal, which would combine the third- and fourth-largest US wireless carriers, is expected to come under serious scrutiny from antitrust regulators, as the Trump administration has fervently opposed AT&T's proposed mega-acquisition of Time Warner.

The agreement marks the culmination of four years of on-again, off-again discussions between T-Mobile and Sprint; this is the third time the two rivals have tried to merge.

With a combined 127 million customers, the two firms are expected to compete directly with Verizon, the US's largest carrier, and AT&T.

"This isn't a case of going from four to three wireless companies — there are now at least seven or eight big competitors in this converging market," Legere said on Sunday.

The agreement involves T-Mobile exchanging 9.75 Sprint shares per unit of T-Mobile. Deutsche Telekom would own 42% of the combined company, while SoftBank, which controls 85% of Sprint, would own 27%. The public would hold the remaining 31%.

Sprint and T-Mobile discussed a deal in November, but talks broke down amid disagreement over who would control the new company. A Wall Street Journal report suggests that SoftBank's founder, Masayoshi Son, may have since become more willing to give up control amid mounting pressure on Sprint to roll out 5G technology.

"This combination will create a fierce competitor with the network scale to deliver more for consumers and businesses in the form of lower prices, more innovation, and a second-to-none network experience — and do it all so much faster than either company could on its own," Legere said in an official statement.

Sprint's CEO, Marcelo Claure, added: "We intend to bring this same competitive disruption as we look to build the world's best 5G network that will make the US a hotbed for innovation and will redefine the way consumers live and work across the US, including in rural America."

The all-stock transaction values Sprint at 0.10256 per T-Mobile share, or $6.62 a share, based on T-Mobile's latest closing price, for a total of about $26 billion.

T-Mobile had a market value of $55 billion as of Friday's close, and the two companies have roughly $60 billion of combined debt.

Sprint's stock has surged 26% since The Journal reported on April 10 that the two companies had rekindled merger talks, while T-Mobile's has risen 8% over the same period.