Democrats in the US Senate flagged a potential T-Mobile US and Sprint merger as a move which could harm consumers and asked for an antitrust review to begin immediately.

In a joint letter, a group of nine senators urged Federal Communications Commission (FCC) chairman Ajit Pai and Department of Justice (DoJ) antitrust chief Makan Delrahim to begin considering the consequences, even before a deal between the operators is agreed.

Amy Klobuchar, the ranking member of the Senate Judiciary Subcommittee on Antitrust, Competition Policy and Consumer Rights, was among those to sign the plea.

The letter comes amid reports Sprint and T-Mobile are hoping to announce a deal during third quarter earnings later this month.

In the letter, the senators argued the result of a T-Mobile and Sprint merger “would very likely be presumptively anticompetitive”, particularly since the pair “appear to be each other’s primary competitor”. Combining the two companies could increase prices, which would harm low-income Americans who have benefitted from Sprint and T-Mobile’s lower-cost offers, the group argued.

“An anticompetitive acquisition would increase prices, burdening American consumers, many of whom are struggling to make ends meet, or forcing them to forego their internet connection altogether. Neither outcome is acceptable,” the group wrote, adding the current four-operator market “has benefited consumers with lower prices, better service, and more innovation. We are concerned that consolidation will thwart those goals.”

The letter echoes concerns raised by Democratic FCC Commissioner Jessica Rosenworcel who, at the monthly commission meeting in September, said Sprint and T-Mobile would need to explain how a deal would benefit consumers and keep prices down, and how having one less bidder in a spectrum auction would be a positive.

Rosenworcel is outnumbered at the FCC: Pai and his Republican colleagues hold a 3-2 majority on the commission.