T-Mobile Inc.’s (TMUS) proposed tie-up with Sprint Corp. (S) has been put on hold by The Federal Communications Commission (FCC).

In a letter, the agency said it paused an 180-day transaction clock at day 55 because it needed more time to review recently submitted materials in support of the $26 billion merger. Submissions up for review include a large,  complex document detailing the benefits a deal would create, information about network expansion plans and an economic modeling report.

“The clock will remain stopped until the applicants have completed the record on which they intend to rely and a reasonable period of time has passed for staff and third-party review,” the FCC said in the letter.

In a separate statement, reported on by Reuters, T-Mobile expressed its confidence that the merger, which is also being reviewed by the U.S. Justice Department, will receive the blessing of regulators. “We are confident that this transaction is pro-competitive, good for the country and good for American consumers,” the company said.

Sprint’s shares fell 1.32% in extended hours trading, while T-Mobile’s stock was flat.

The merger of the third- and fourth-largest U.S. wireless carriers has proved to be a drawn out affair. The companies finally agreed terms on a deal after four years of on-and-off talks in April. (See also: Sprint, T-Mobile Looking to Sign Merger Deal Next Week: Report.)

T-Mobile and Sprint plan to develop a next-generation 5G network capable of providing faster speeds, more capacity and lower response times. By joining forces, the two companies are hopeful that they can cut costs and better compete with the two largest wireless carriers, Verizon Communications Inc. (VZ) and AT&T Inc. (T), during a period of industry-wide technological change.

However, getting regulatory clearance isn’t expected to be easy. Critics are concerned that reducing the number of major wireless carriers in the nation from four to three will ease competitive pressure and subsequently lead to higher prices. (See also: Verizon Valuation Caps Upside Potential: Barclays.)