Making the wearing of face masks in shops in England compulsory could "trigger abuse" of staff, unions and businesses have warned.
Union Usdaw said workers should not be made to enforce the rule if it becomes law as it could make them targets.
And the Federation of Small Businesses said masks could make some "happier to shop", but others more nervous.
On Monday, Boris Johnson said people in England "should be wearing" face masks or other coverings inside shops.
The prime minister said the government would decide in the next few days if "tools of enforcement" were needed.
Waterstones boss James Daunt told the BBC's Today Programme that making people wear masks in shops seemed "a perfectly reasonable thing".
But the book chain boss also said it "would not be right" to ask staff to enforce the rule.
"There are a tiny, tiny minority of people who will be confrontational over it and it is not the position of shop workers to enter into that situation," he said.
"We shouldn't put ourselves in confrontational positions, but I think we can, as retailers, if we are requested to do so, clearly tell everybody it is a sensible thing to do."
The union Usdaw said workers were already dealing with more abuse than normal and urged the government to clarify its policy on face coverings in shops.
"It should never fall on shop workers to enforce the wearing of face coverings," warned Paddy Lillis, Usdaw general secretary.
Mike Cherry, national chair of the Federation of Small Businesses, added: "We must be careful that this does not become a trigger for abuse against shop owners and their employees.
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"[This policy] is likely to make some people happier to shop, and make others more nervous."
Linda Bauld, professor of public health at the University of Edinburgh, told the Today programme that evidence now suggests that wearing masks in confined spaces makes a difference.
"Essentially infections are spread by droplets and they're relatively large when they come out in a cough or a sneeze but they become much smaller as they travel through the air.
"So the job of a cloth covering isn't so much to protect the wearer but to block the source of infection," she said.
She said leaders should lead by example by being seen wearing face coverings and should explain clearly why it is needed or required.
"Enforcement needn't be heavy-handed. Requiring people to wear masks in shops will give that little extra nudge and make it much clearer for the general public," she said.
'Basic good manners'
Last week, Prime Minister Boris Johnson appeared for the first time in public in a mask and hinted that stricter rules were coming in England.
But on Sunday, Cabinet Office Minister Michael Gove said there were no immediate plans to make masks in shops compulsory.
However, on Monday Mr Johnson said: "I think people should be wearing [face masks] in shops.
"And, in terms of how we do that whether we make it mandatory or not, we'll be looking at the guidance - we'll be seeing a little bit more in the next few days."
Only 7% of shoppers say they have "complete faith" in current protective measures put in place by retailers, according to retail solutions firm Cennox.
Its research suggested just over half of shoppers that had visited stores found measures inadequate or poorly managed.
"For the foreseeable future, safety is the new gold standard for retail loyalty," said Cennox boss Nick Cockett. "Shoppers will vote with their feet if they don't feel protected."