The boss of supermarket chain Sainsbury's has said the company will enforce mask-wearing and shopping alone in its stores.
In a statement, Simon Roberts said the chain has written to customers to highlight the importance of keeping others safe, while it has also "significantly reduced" the number of customers allowed into its stores at any one time.
"I've spent a lot of time in our stores reviewing the latest situation over the last few days and on behalf of all my colleagues, I am asking our customers to help us keep everyone safe," Mr Roberts said.
"The vast majority of customers are shopping safely, but I have also seen some customers trying to shop without a mask and shopping in larger family groups.
"Please help us to keep all our colleagues and customers safe by always wearing a mask and by shopping alone.
"Everyone's care and consideration matters now more than ever."
Sainsbury's said it will have trained security guards at the entrances to its stores to challenge those shopping in groups or not wearing a face covering.
Face coverings are currently mandatory in shops and other enclosed places in Northern Ireland, however there has been some confusion as to who is responsible for enforcement and how this can be practically done.
PSNI Assistant Chief Constable Alan Todd said on Monday that officers had been visiting businesses and speaking to owners and "we continue to support them by our presence and the patrols we undertake".
"Clearly we would wish everyone complies with the regulations and wears their face mask in shops, unless they have a medical excuse for not doing so," he added.
"That will continue to be the case."
Supermarket chain Asda has also taken steps to clamp down on those not wearing face coverings in its stores by deploying "safety marshals" to ensure that all customers stick to the rule.
The policy means that anyone spotted without a covering will be offered a pack of disposable face masks. which the customer has to purchase at the end of their shop.
Tesco, meanwhile, has said it is limiting customer numbers in stores and reminding customers to wear face coverings.
"We have clear signage explaining this, and we have packs of face coverings available for purchase near the front of our stores for any customers who have forgotten them," a spokesperson said.