New York (CNN Business) Walmart has monopolized the grocery business across the United States and should be forced to sell off some of its stores, an advocacy group charged in a scathing report released Thursday.

Walmart is the nation's largest grocer. It controls around a fifth of the grocery industry nationwide. The company's strength in some corners of America is overwhelming and has stifled competition, according to an analysis by the Institute for Local Self-Reliance , a nonprofit group that criticizes Walmart, Amazon and other large companies.

The Institute for Local Self-Reliance "challenges concentrated economic and political power" and advocates for widely-dispersed responsibilities in local communities. It is funded mostly by foundation grants, such as Ford and Wallace Global, said Stacy Mitchell, co-executive director of the group.

In 43 US metropolitan areas and 160 smaller American markets, Walmart controls 50% or more of grocery sales, according to the report. In 38 of these regions, Walmart's share of the grocery market is 70% or higher, the group says.

"No other corporation in history has ever amassed this degree of control over the US food system," said Mitchell, the author of the report.

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