Shares of Sprint Corp. (S - Get Report) and T-Mobile US  (TMUS - Get Report) fell sharply in premarket trading Wednesday after a published report suggested the Justice Department won't approve their merger as currently structured.

Sprint shares fell 6.7% in premarket trading to $5.61, while T-Mobile slumped 2.8% to $72.

Justice Department officials are balking at approving the $26.5 billion acquisition of Sprint by T-Mobile over antitrust concerns, The Wall Street Journal reported, citing unnamed sources.

In a meeting earlier this month, Justice Department staff members laid out their concerns with the all-stock deal and questioned the companies' arguments that the combination would produce important efficiencies for the merged company, the people told the Journal.

The transaction was first announced nearly a year ago.

T-Mobile CEO John Legere responded to Journal article in a tweet, saying, "The premise of this story, as summarized in the first paragraph, is simply untrue. Out of respect for the process, we have no further comment."