All three Federal Communications Commission (FCC) commissioners voted in favor of the deal, as well as Sprint's related bid to buy out the shares of wireless company Clearwire Corp. that it does not already own, said the sources who spoke anonymously because the approval has not yet been announced publicly.

SoftBank's $21.6 billion deal to buy 78 percent of Sprint would mark the largest-ever overseas acquisition by a Japanese company. In filings with the FCC, Sprint, Clearwire and SoftBank had said they hoped to close both deals on July 8 or 9.

FCC acting Chairwoman Mignon Clyburn and Democratic Commissioner Jessica Rosenworcel cast their approval votes last week, while the required third, and final vote from Republican Commissioner Ajit Pai was delayed until Wednesday by negotiations over the wording used to address Sprint's ownership of airwaves, according to one of the sources.

FCC, Sprint and Clearwire spokesmen declined comment. SoftBank could not be immediately reached for comment.

The FCC's review, running more than 70 pages long, focused on whether the deals were in the U.S. public interest.

SoftBank had earlier received approvals from U.S. antitrust and national security regulators, as well as Sprint shareholders.

Sprint already owns a majority stake in Clearwire, but several minority shareholders had launched a boisterous campaign seeking a higher bid, fueled by a rival offer from satellite TV service provider Dish Network Corp.

Crest Financial, a large minority shareholder of Clearwire, which had pushed against Sprint and thrown wrenches into the FCC's review of the SoftBank deal, dropped its opposition on Wednesday after Sprint raised its bid.

Dish had also aggressively bid for Sprint against SoftBank, forcing the Japanese company to increase its offer. Dish pulled out of both bidding wars last month.

Clearwire is an important part of SoftBank's interest in Sprint because Clearwire holds a large amount of wireless airwaves that would help Sprint compete with bigger rivals.