The Journal says Justice Department antitrust staff, which is reviewing the takeover, questioned the companies’ reasoning for it in a meeting this month. The report cited people familiar with the matter.
Wall Street has grown more skeptical of the merger being completed in recent months. House Democrats grilled the companies in a hearing in February.
The Obama administration rebuffed the companies’ earlier effort to merge, as well as an attempted deal between AT&T and T-Mobile.
Messages to T-Mobile, Sprint and DOJ were not immediately answered.
Sprint shares are down more than 9 percent in after-hours trading. T-Mobile stock is down 5.5 percent.