Scientists have cautiously backed Boris Johnson's hints that face coverings may be made compulsory in English shops, but warned that masks are not a substitute for social distancing.
The Government insisted early on in the pandemic that masks were not necessary for use by the general public when out and about, before making coverings compulsory on public transport and in hospitals in England.
But on Friday, the Prime Minister hinted at a change in policy, telling a public question-and-answer session that he wanted to be “stricter” on making Brits cover their faces in confined spaces.
Mr Johnson said: “We are looking at ways of making sure that people really do have face coverings in shops, for instance, where there is a risk of transmission.”
Epidemiologist Professor David Heymann, of the London School of Hygiene and Tropical Medicine, said masks should be worn by “all people in a situation where no one can physically distance to prevent infection of others”.
He said: “Face masks protect others from infection by catching virus-containing droplets when a person who is infected and has a high level of virus in the nasal passage speaks, shouts, sings, coughs or sneezes.
“Face masks should be worn when physical distancing cannot be assured from others such as by carers in care homes and by people serving others who are physically distancing but who, because of their work, cannot physically distance from them.
“They should also be worn by all people in a situation where no one can physically distance to prevent infection of others – especially in closed spaces such as public transport.
“Face masks do not substitute for physical distancing if physical distancing is possible, and they do not protect the wearer from infection unless they are worn as part of personal protective equipment that also protects the eyes, a potential site of infection.”
Dr Paul Hunter, professor of medicine at the University of East Anglia, said the Prime Minister was “right” to be reviewing England’s position on face coverings, but it is unlikely the scientific debate on the issue will be resolved anytime soon.
He added: "If it is mandated to wear them (coverings) in shops, this raises the issue of whether they should be mandated in other contexts.
"The most important thing, however, is that anyone wearing a mask must not assume that they are automatically protected. People should still practice distancing and continue to wash their hands.”
Professor Adam Finn of the University of Bristol said: “The more efficient the face covering is at catching the droplets, the better it will work.
“So if you are in a shop and everyone else is wearing a mask, you should feel safer than if they aren’t.”
But UCL epidemiologist Dr Antonio Lazzarino said he was worried masks are “a pretence to ease the lockdown to help the economy”.
He added: “This may well happen at the expense of people’s health. Lockdown is the only measure that is proven to work.”
Welsh First Minister Mark Drakeford said he did not think the evidence on wearing face coverings was “decisive” yet.
“I agree that as time goes on there is more emerging on the side of supporting face coverings in public,” he said.
“But I don’t think it is decisive yet. When the debate reaches a point, if it does, when the advice changes, then the position will change in Wales.”
Office for National Statistics figures from the first week of July suggested more than 50 per cent of adults in Britain who left their home in the week before being surveyed had worn a face covering, up from 43 per cent the week before.
Former Health Secretary Jeremy Hunt, a Conservative, called for simple Government messaging and said people should wear face coverings in shops.
The chairman of the Commons Health and Social Care Committee told BBC Radio 4’s Today programme: “I’m afraid I do go all ‘nanny’ on that one.
“I understand the public health advice, which is that if there’s a risk of being less than two metres close to someone then you should wear it, but if not you don’t have to.
“But it doesn’t answer the basic question, which is: ‘If I’m going shopping, should I wear a face mask or not?’
“And I think with public health advice in a pandemic you just need simplicity, so I would favour saying we should wear face masks in shops.”
The World Health Organisation said earlier this week that there is “emerging evidence” that coronavirus could be spread through particles travelling through the air.
And Royal Society president Professor Venki Ramakrishnan said everyone should wear a face covering in public to reduce the risk of a second wave of coronavirus.