Supermarkets will be unable to sell items like clothes during the 17-day Covid firebreak lockdown in Wales.
First Minister Mark Drakeford said it would be "made clear" to them they are only able to open parts of their business that sell "essential goods".
Many retailers will be forced to shut but food shops, off-licences and pharmacies can stay open when lockdown begins on Friday at 18:00 BST.
Retailers said they had not been given a definition of what was essential.
The Association of Convenience Stores and the Welsh Retail Consortium have written urgently to the first minister, expressing alarm over the new regulations.
Sara Jones, head of the Welsh Retail Consortium, said they wanted the Welsh Government to abandon the "essential items" rules.
"Compelling retailers to stop selling certain items, without them being told clearly what is and what isn't permitted to be sold, is ill-conceived and short-sighted," she said.
Welsh Conservative Andrew RT Davies tweeted: "The power is going to their heads."
But Plaid Cymru's Helen Mary Jones said "smaller businesses should not be put at an unfair disadvantage during the firebreak lockdown".
Business leaders say companies in Wales have been given just hours to finalise plans for the firebreak lockdown, which ends at midnight on 9 November.
Mr Drakeford told a Senedd committee on Friday that "in the last lockdown, people were reasonably understanding of the fact that supermarkets didn't close all the things that they may have needed to".
"I don't think people will be as understanding this time.
"We will make sure there is a more level playing field in those next two weeks."
The first minister was responding to Conservative Member of the Senedd Russell George, who said it was "unfair" to force independent clothing and hardware retailers to close while similar goods were on sale in major supermarkets.
"It felt very wrong and disproportionate to the small businesses," Mr George said.
Mr Drakeford said: "We will be making it clear to supermarkets that they are only able to open those parts of their business that provide essential goods to people.
"And that will not include some of the things that Russell George mentioned, which other people are prevented from selling."
There is no precise list of non-essential goods in the law coming into force on Friday, but any business selling goods or services for sale or hire in a shop will have to close.
But there are exceptions for food retailers, newsagents, pharmacies and chemists, bicycle shops, petrol stations, car repair and MOT services, banks, laundrettes, post offices, pet shops and agricultural supplies shops.
Under the law firms conducting a business that provides a mixed set of services will be allowed to open if they cease conducting the service that must close.