The Federal Communications Commission has approved the $26.5 billion combination of Sprint and T-Mobile on a 3-2 party-line vote.
The wireless merger still faces opposition from a coalition of state attorneys general, who argue the deal is bad for competition. The companies won't merge while litigation persists.
The antitrust trial is scheduled to start in New York in December, an unusual situation given that the Trump administration's Justice Department approved the deal.
The FCC chairman, Ajit Pai, a Republican, backed it months ago citing the companies' promise to build out a next-generation 5G network to many rural areas, improving internet access.
The Democratic commissioners say going from four to three major wireless companies will mean higher prices for consumers. They say it will be difficult to enforce promises made by T-Mobile and Sprint.