After years of speculation and on-again, off-again talks, the nation's third- and fourth-largest wireless carriers unveiled a plan to merge over the weekend.

T-Mobile and Sprint would join forces in an all-stock transaction worth a reported $26 billion. Deutsche Telekom, T-Mobile's parent, would own nearly 42 percent of the combined company and a majority of seats on its board.

SoftBank Group, the controlling shareholder of Sprint, would own about 27 percent of the new company, with the remainder owned by the public.

If approved by antitrust regulators, the company — which would retain the T-Mobile name and be led by T-Mobile CEO John Legere — would leap AT&T to the No. 2 position in the U.S. wireless market, behind only Verizon.

"As industry lines blur and we enter the 5G era, consumers and businesses need a company with the disruptive culture and capabilities to force positive change on their behalf," Legere said in a statement.

The carriers repeatedly discussed a merger in recent years as a way to ease costs and compete with larger companies in a mature wireless market. Both sides were close to an agreement late last year until SoftBank founder Masayoshi Son determined he did not want to cede control of the company to the German telecom.

This week, however, Sprint CEO Marcelo Claure indicated that was no longer a concern.

"We’re happy with the percentage of our ownership and happy to put the company in the hands of such a strong management," Claure told The New York Times.

Consolidating the domestic wireless market from four major carriers to three, meanwhile, is certain to prompt a thorough examination by the U.S. Department of Justice, and observers were not exactly optimistic that the merger would survive an antitrust review.

As the companies negotiated last fall, the FCC determined that the current wireless market provided effective competition in the U.S., and reports said DOJ staff were likely to oppose a merger.

"The message from our regulatory contacts was simple — 'this won’t be easy,'" Wells Fargo Senior Analyst Jennifer Fritzsche wrote in a research note.

Company officials, however, touted the potential economic benefits of combining the carriers, particularly as operators worldwide prepare to roll out 5G technology.

Executives said combining the assets of both companies would force its rivals to invest in next-generation technologies as well, which would foster competition and job creation.

They also warned that neither company alone would be able to establish nationwide 5G with the "breadth and depth required to fuel the next wave of mobile internet innovation."

"This isn’t a case of going from 4 to 3 wireless companies — there are now at least 7 or 8 big competitors in this converging market," Legere argued. "And in 5G, we’ll go from 0 to 1."

Fritzsche added that despite the potentially steep regulatory hurdles, a combined company would offer "deep spectrum assets, a well-perceived brand and a management team that has successfully integrated past acquisitions"

"We will be very clear that if [T-Mobile] gets this done — it would be in an extremely strong position in our view," Fritzsche wrote.