WASHINGTON—The Justice Department filed an antitrust lawsuit challenging U.S. Sugar’s proposed purchase of rival Imperial Sugar, arguing the tie-up would lead to higher prices for refined sugar and food-and-beverage staples for consumers.

The suit, filed Tuesday in a Delaware federal court, is the fourth major deal challenge in recent months from the department, which has been following through on a Biden-era enforcement pledge to take a harder line against industry consolidation.

Privately-held...

WASHINGTON—The Justice Department filed an antitrust lawsuit challenging U.S. Sugar’s proposed purchase of rival Imperial Sugar, arguing the tie-up would lead to higher prices for refined sugar and food-and-beverage staples for consumers.

The suit, filed Tuesday in a Delaware federal court, is the fourth major deal challenge in recent months from the department, which has been following through on a Biden-era enforcement pledge to take a harder line against industry consolidation.

Privately-held U.S. Sugar, headquartered in Clewiston, Fla., farms more than 200,000 acres of sugar cane in the state. In March it announced an agreement to purchase Texas-based Imperial Sugar, whose operations include a large refinery in Georgia, from Dutch-based Louis Dreyfus Company B.V. The deal was valued at about $315 million, according to the Justice Department lawsuit.

The department alleged the deal would leave most sales of refined sugar across the Southeast in the hands of only two producers, which it said was an unlawful reduction in competition that would spur higher prices for a product vital to the nation’s food supply.

“U.S. Sugar and Imperial Sugar are already multibillion-dollar corporations and are seeking to further consolidate an already cozy sugar industry,” said the department’s new antitrust chief, Jonathan Kanter, who won Senate confirmation last week.

Mr. Kanter said the deal “substantially lessens competition at a time when global supply chain challenges already threaten steady access to important commodities and goods.”

U.S. Sugar said it disagreed with the department’s conclusions and intends to litigate the matter.

“The facts will ultimately show that U.S. Sugar’s acquisition of Imperial Sugar will result in increased production and distribution of refined sugar, provide a more secure sugar supply for American farmers, food producers and consumers, and protect American jobs,” the company said. “This transaction will improve supply chain logistics and will not result in higher prices or any harm to customers and consumers. We look forward to making our case in court.”

Tuesday’s lawsuit marks the latest sign of the Biden administration’s tougher stance on antitrust enforcement. Earlier this year it blocked a more than $30 billion insurance-industry merger between Aon PLC and rival Willis Towers Watson PLC. It is currently challenging a partnership between American Airlines Group Inc. and JetBlue Airways Corp.

as well as a deal that would combine two top publishers, Penguin Random House and Simon & Schuster.

U.S. Sugar sells its refined sugar through United Sugars Corp., a marketing cooperative owned by a handful of producers. Aggressive competition between United and Imperial Sugar has resulted in better wholesale prices for businesses large and small, as well as for consumers, the Justice Department’s lawsuit alleged.

The department said the case was “not a close call.”

The case potentially could raise issues related to a sugar-industry regulatory program run by the U.S. Department of Agriculture.

According to the lawsuit, U.S. Sugar and Imperial have argued the program gives the USDA the ability to prevent the kinds of harms the Justice Department alleges.

The department disagreed, arguing USDA regulation wasn’t a substitute for antitrust enforcement and wouldn’t protect grocers, food and beverage manufacturers or consumers from diminished competition.

Write to Brent Kendall at brent.kendall@wsj.com