Chief executive Simon Roberts said: “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.
“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.”
After concerns from unions about worker safety Boris Johnson said people needed to “do the right thing” by adhering to social distancing when out.
“We need to enforce the rules in supermarkets. When people are getting takeaway drinks, in cafes, then they need to avoid spreading the disease there, avoid mingling too much,” he said.
Supermarket chain Morrisons said on Monday that customers who refuse to wear a mask without a medical exemption will be told to leave stores.
Chief executive David Potts said: “Those who are offered a face covering and decline to wear one won’t be allowed to shop at Morrisons unless they are medically exempt.
“Our store colleagues are working hard to feed you and your family, please be kind.”
Matt Hancock, the health secretary, applauded Morrisons in his Downing Street briefing, “That’s the right approach, and I want to see all parts of society playing their part in this.”
“Stronger enforcement is necessary, and I’m delighted that the police are stepping up their enforcement. But it isn’t just about the government and the rules we set, or the police and the work that they do. It’s about how everybody behaves.”
But one senior police leader told the Guardian that they won’t be enforcing mask-wearing in supermarkets due to resources.
They said: “Do people really want the police telling you: ‘that’s not above your nose’? There are no extra officers. Everything else [crime] is still happening. “Where is the greater risk: do you put two people in a supermarket not wearing masks before a woman suffering domestic violence?
“You need clearer, consistent messaging, not new rules and more enforcement.”
Vaccines minister Nadhim Zahawi said the government is “concerned” shops are not enforcing rules strictly enough.
“Ultimately, the most important thing to do now is to make sure that actually enforcement – and of course the compliance with the rules – when people are going into supermarkets are being adhered to,” Mr Zahawi told Sky News.
“We need to make sure people actually wear masks and follow the one-way system,” he said.
Shopworker’s union Usdaw general secretary Paddy Lillis said: “Retail staff are working with the public every day and not only suffer increased abuse, but are deeply worried about catching Covid-19.
“Where safety measures are agreed, retailers need to make sure that they are being followed consistently, in every store.
“We are also very concerned by reports that too many customers are not following necessary safety measures like social distancing, wearing a face covering and only shopping for essential items.
“Many retail workers are at a greater risk of catching the virus and bringing it home to their families.
“These key workers must be valued, respected and protected.”