Jersey's council of ministers has agreed a policy to allow future retail development, after a study by Experian indicated that stores on the island were over-trading compared with UK counterparts.
However, existing traders disagree with the report's findings and fear a new supermarket - possibly to be built in an out-of-town location - will drain trade from existing stores, particularly convenience stores around the capital St Helier.
They also argue there will be nobody on the island to staff any new stores because Jersey has full employment.
The Chamber of Commerce, which represents 500 businesses on Jersey, including retailers, said it would oppose plans to build any new stores and demanded an inquiry.
Chamber president Kevin Keen said: "The politicians clearly want a new retail site, but we can't see any grounds for one.
"We're calling for an independent economic and environmental assessment of the impact it would have on Jersey and on local businesses."
Keen said a separate study commissioned by the Chamber of Commerce showed that Experian's findings were flawed. "If anything, stores are under-trading," he said.
But minister of economic development Senator Philip Ozouf said new retail outlets would be allowed only if analysis showed they could bring benefits to the island. "We have set out a solution that should benefit both shoppers and shopkeepers. Consumers will be pleased because it should allow for greater competition in the retail sector and more choice. Retailers will be encouraged by the incremental approach, which allows the impact of new space to be monitored."
Food retailers already operating on Jersey include Le Riche, which runs Checkers outlets and the old Safeway store, M&S, Channel Islands Co-operative and Spar.
There is speculation about the possible siting of a new supermarket. Jersey Royal Potato Marketing has revealed it is in talks with UK supermarkets to sell a packhouse at Maufant. Another possible site is the waterfront near St Helier.