The Federal Trade Commission on Monday sued to block Kroger’s $25 billion bid for rival Albertsons , throwing into uncertainty the fate of the largest supermarket deal in history.

In a lawsuit filed in federal court in Oregon, the FTC said the deal would lead to higher food prices and harm union workers’ bargaining power, and asked a court to block the companies from closing their deal on antitrust grounds. The companies’ plan to address the government’s concerns by selling hundreds of stores in Washington, Colorado and other states won’t solve the problem, the FTC said.

“Kroger’s acquisition of Albertsons would lead to additional grocery price hikes for everyday goods, further exacerbating the financial strain consumers across KR 0.57% ACI -0.75% the country face today,” said Henry Liu, director of the FTC’s bureau of competition.

A Kroger spokeswoman said the FTC’s decision makes it more likely that America’s consumers will see higher food prices and fewer grocery stores, while strengthening nonunionized retailers. An Albertsons spokesman said the combined company would be a stronger competitor against Amazon, Walmart and Costco...

It also signals a high-water mark for the role of labor in antitrust analysis, said William Kovacic, a former FTC chairman now at George Washington University. The FTC and Justice Department, which share antitrust enforcement duties, now probe whether mergers will lead to bigger employers with excessive leverage over workers.

The FTC’s claim that Kroger-Albertsons would have more power to neutralize the threat of a union boycott or strike is uncommon, Kovacic said. “Showcasing it directly [in an antitrust case] is new,” he said...

Regional leaders for the United Food and Commercial Workers International Union, the biggest U.S. union representing grocery workers, have also opposed Kroger’s deal for Albertsons, saying it would hurt workers, shoppers and the food-supply chain. Some union leaders have expressed concerns about the viability of stores the companies divest.

The FTC said Monday that Kroger and Albertsons would have more leverage to impose subpar terms on union grocery workers that slow wage increases, worsen benefits and potentially degrade working conditions...

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