Morrisons has confirmed customers who refuse to wear a mask without a medical exemption will be told to leave stores from today.

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.”

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.

Tesco and Sainsbury’s said safety remains their “highest priority”, and that customers were given regular reminders to follow the rules.

A spokeswoman for Sainsbury’s said: “We continue to have a range of measures in place to keep customers and colleagues safe in our stores.

“We have greeters outside our supermarkets and busy convenience stores to limit the number of customers coming into stores and continue to remind customers to wear face coverings and shop alone if they are able to.

“We also have posters and regular Tannoy announcements.”

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.”

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.”