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Staff at the Department of Justice's antitrust division have recommended that the agency sue to block T-Mobile's (TMUS - Get Report) proposed $26 billion acquisition of Sprint (S - Get Report) , according to a Reuters report.

The news sent both stocks lower in premarket trading Wednesday. 

The department will make a final decision in about a month, Reuters reported. The final decision on whether to allow the merger to go forward rests with political appointees at the antitrust division, which is headed by Makan Delrahim. 

Earlier this week, the chances of the merger getting clearance received a boost after Federal Communications Commission Chairman Ajit Pai came out in favor of the merger after the third- and fourth-largest wireless carriers in the country offered concessions to get the deal done. 

The FCC has been drafting an order it plans to circulate for approval in the coming weeks, Reuters reported. 

Shares of Sprint were down 2.5% Wednesday to $7.04, while T-Mobile fell 0.7% to $76.61.