The U.S. Justice Department filed an antitrust lawsuit to block Anheuser-Busch InBev NB’s (NYSE: BUD) proposed $20.1 billion acquisition of Grupo Modelo (MM: GMODELOC), saying the deal would reduce competition in the beer market.

The suit, filed on Jan. 31, said that Anheuser’s proposed total ownership of Modelo would result in consumers paying more for beer and having fewer new products to choose from.

Even a small increase on the price of beer could result in billions of dollars that could harm American consumers, the department said. Americans spent at least $80 billion on beer last year.

The department filed the lawsuit in the U.S. District Court for the District of Columbia.

Anheuser and Modelo are the first and third largest beer firms, respectively, together accounting for 46 percent of annual beer sales. MillerCoors is the second largest. Anheuser owns Budweiser, Bud Light,  Michelob, Shock Top, Rolling Rock and other beer brands. Modelo owns stakes in Corona, Model and Pacifico, as well as other brands.

Anheuser agreed to buy Modelo in June 2012.