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Speaking on a visit to the London Ambulance HQ on Monday, the prime minister said coverings have “have real value in confined spaces” and that people “should” wear them in shops.
He told reporters there was “growing” evidence face coverings could limit the spread of Covid-19.
But he went on to say his government had yet to make a ruling on whether they should be made compulsory.
His official spokesperson, meanwhile, has said a formal review is under way into whether masks should be made mandatory in public places in England.
At a briefing with reporters on Monday, he said there is further evidence about the “benefits of face coverings in some environments indoors”.
It comes after Cabinet Office minister Michael Gove added to the mixed messaging by saying on BBC One’s Andrew Marr Show that coverings would not be enforced in shops but that wearing them was “basic good manners”.
Johnson said: “They have a great deal of value in confined spaces where you’re coming into contact with people you don’t normally meet.
“The scientific evaluation of face coverings and their importance on stopping aerosol droplets, that’s been growing, so I do think that in shops it is very important to wear a face covering if you’re going to be in a confined space and you want to protect other people and receive protection in turn.”
Asked if it was his personal view that coverings should be worn, he said: “Yes. Face coverings, I think, people should be wearing in shops, and in terms of how we do that – whether we will be making that mandatory or not – we will be looking at the guidance. We will be saying a little bit more in the next few days.”
The PM said coverings had “real value” and were “a kind of extra insurance” for the public.
His words echoed those of justice secretary Robert Buckland who said coverings should be worn and that they would boost “confidence”.
Johnson added: “We will be looking in the next few days about exactly how, with what tools of enforcement, we think we want to make progress [over coronavirus].
“As the virus comes down in incidence and we have more and more success, I think face coverings are a kind of extra insurance we can all use to stop it coming back and stop it getting out of control again.
“To be absolutely clear I do think that face coverings do have a real value in confined spaces and I do think the public understand that.”
The PM’s official spokesman added: “A formal review is taking place and we will be discussing it with scientific advisers this week and setting out a final position in the next few days.”
Asked whether there is new evidence behind Johnson’s thinking, the spokesman said: “I’ve always said we would keep this under review and so have the experts who advise the government.”
Johnson also underlined that people “should start to think about getting back to work” if their employers have made their workplaces safe against coronavirus.
Gove had also called on the public to return to work to “fire up” the UK economy, but acknowledged this might not be appropriate in all circumstances.
Businesses have complained that differing messages are confusing, and that the official guidance continues to encourage people to work from home where possible.
Johnson said: “What I want to see is people who have been working from home for a long time, now talking to their employers, talking to their places of work, about the steps that have been taken, and looking to come back to work in a safe way,” he said.
“I think that’s got to be the key thing. Where businesses have made a huge effort to make the workplace safe I do think people should start to think about getting back to work – but provided we all continue to follow the precautions.”
This article originally appeared on HuffPost and has been updated.