State authorities led by New York Attorney General Letitia James will file a lawsuit as soon as Tuesday to kill the proposed $26 billion merger, according to the reports.

James has scheduled a press conference at 1700 GMT in New York. Her office did not immediately respond to a query from AFP.

Shares of Sprint tumbled 7.2 percent to $6.49 in late-morning trading, while T-Mobile shed 2.4 percent to $74.87.

T-Mobile and Sprint are respectively the third- and fourth-largest wireless carriers in the United States with a combined 131 million subscribers, bringing a merged company closer to the top two companies, Verizon and AT&T.

Sprint, majority owned by Japan's SoftBank, and T-Mobile, a unit of Germany's Deutsche Telekom, have said the deal is needed to compete in developing the next-generation 5G network.

The companies have agreed not to raise rates for three years and will divest from the Sprint subsidiary Boost Mobile.

But consumer advocates have argued that permitting the merger would lead to higher prices throughout the industry.

The Federal Trade Commission approved the transaction last month with conditions, but the Justice Department is still reviewing the deal.