The US Supreme Court ­on Monday struggled to decide whether setting ­minimum prices for retail goods is always illegal, with several justices warning that any change in present price-fixing rules could transform the US retail industry and the fortunes of discounters such as Wal-Mart.

The issue before the court is whether to overturn a century-old precedent, that bars manufacturers and retailers from striking agreements setting minimum prices by making such deals an automatic violation of America’s antitrust laws.