Boris Johnson has urged the public to respect coronavirus regulations when out grocery shopping and buying takeaway coffee, amid "deep" concerns from food retail workers over their safety.
Shopworkers' union Usdaw said that it had been "inundated" with complaints from its members – who are classed as key workers – and called for supermarkets to revert to more stringent in-store measures.
Retail bosses have also suggested that further police support is needed to help enforce measures, which they say are leading to an increase in abuse towards staff.
The prime minister 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.
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."
Andrew Opie, director of food and sustainability at the British Retail Consortium (BRC), said: "While enforcement of face coverings is the responsibility of the police, retailers continue to do what they can to encourage their use throughout stores.
"Sadly, this has led to a sharp rise in incidents of violence and abuse against shop workers, which is why it is essential police support the work being done by retailers."
England's chief medical officer, Professor Chris Whitty, said that supermarket staff would not be initially prioritised for vaccinations unless they also fell into the highest vulnerability categories.
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."
Tesco and Sainsbury's said safety remains their "highest priority", and that customers were given regular reminders to follow the rules.
Sainsbury's later confirmed it will post trained security guards at shop entrances to "challenge" any customers not wearing a mask or shopping in groups.
Chief executive Simon Roberts: "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."
A spokeswoman for Sainsbury's added guidance remained "shop alone if you can" and customers with children or carers would not be refused entry.
Tesco said it was still enforcing social distancing practices in store but was not looking yet at reintroducing measures such as one-way aisles.
"The safety of our customers and colleagues is our top priority and we already have extensive social distancing measures in our stores to ensure everyone can shop safely with us," the store said on Twitter.
"We are asking all our customers to wear a face covering when visiting our stores and have prominent signs in place to inform customers of the rules.
"However, there may be some customers who are unable to wear a face covering for medical or safety reasons and we have asked our colleagues to respect that and to not challenge them directly."
Local authorities have also promised to work with the Government to enforce rules in supermarkets.
A spokesperson at the Local Government Association (LGA) said: "We are aware that an ongoing concern among councils and enforcement officers relates to levels of social distancing, the use of face masks and infection control compliance at supermarkets and are pleased that the Government is currently looking into this.
"We will continue to work with councils and Government to ensure that any changes brought in to increase safety in our communities are done so effectively."