All employers in Wales are being told to do their best to keep workers two metres (6ft) apart or face a fine, as a new law comes into force.
Companies are being asked to take “reasonable measures” to keep staff healthy during the coronavirus crisis.
The Welsh Government believes the move is necessary after some employees complained they were not social distancing at work.
Police and councils will have the power to enforce the rules.
They will be able to issue fines of between £60 and £120.
But business organisation CBI Wales said the way the law had been introduced had caused companies “great difficulties”.
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First Minister Mark Drakeford said the new rules are not an “absolute ban” on people working closer together than two metres.
Instead, he urged employers to take “all reasonable measures” to ensure the health and safety of workers, adding that the law will largely be “self-policing”.
Concerns have centred on a number of companies, including an outlet of pizza chain Domino’s after it denied sacking employees who made complaints.
Angharad Maddocks claimed she was dismissed in the Llanelli branch after complaining social distancing and hygiene guidance was not followed.
“Even on the make-line there’s no masks given,” she said, describing 20 people in the store “crammed together”.
How will the law work?
The rules require all workplaces, businesses and organisation to take “all reasonable measures” to ensure a distance of two metres is maintained between people on their premises, and those waiting to enter.
Police and councils will be able to enforce the moves, which came into force in the early hours of Tuesday morning.
Guidance for how the law will operate says there will be circumstances where it is not possible to take such measures.
“But employers will be expected to undertake an assessment before concluding there are no reasonable measures that they can take, and there are no blanket exemptions form this legislation,” the guidance says.
Breaches can attract fines of £60 for a first offence, reducing to £30 if paid within 14 days, and £120 for second and further breaches.
The rules amend the original stay-at-home law that was introduced almost two weeks ago – and use the same enforcement powers and fines.
The law stated people should only go out for things such as shopping for basic necessities, exercising once a day and getting medical attention.