Four states joined a lawsuit to block a proposed merger between Sprint and T-Mobile US, as a judge set a tentative trial date for October.

Hawaii, Massachusetts, Minnesota and Nevada signed on to a complaint filed earlier this month by legal officials from nine other states and the District of Columbia, which alleges the deal would harm competition and consumers.

Maura Healey, Massachusetts attorney general, said in a statement the decision followed a year-long investigation which concluded the tie up would “give the new company the power to raise prices, significantly reduce competition for customers, lower quality, and cost thousands of retail workers their jobs”.

Filings with the Federal Communications Commission (FCC) show attorneys general in at least seven other states have taken up investigations into whether the deal would violate any laws, but have not joined the lawsuit to stop the deal.

FCC Chairman Ajit Pai in a speech blasted the states’ attempt to block the deal as “misguided”, arguing the move would prevent rural consumers from gaining access to fast mobile broadband. Pai came out in favour of the merger in May.

Trial arguments are tentatively scheduled to begin on 7 October. However, in a hearing, a judge noted action by the Department of Justice (which has yet to issue a decision on the deal) could affect the proceeding.