Scientists have cautiously welcomed Boris Johnson’s suggestion that face coverings may become mandatory in shops in England.

The Prime Minister said on Friday that he wanted to be “stricter” on insisting people wear coverings in confined spaces where they are meeting people they do not normally see.

Face coverings are currently compulsory on public transport and in hospitals in England, but are now mandatory in shops in Scotland.

The UK Government insisted early on in the pandemic that masks were not necessary for use by the general public when out and about.

But the PM signalled a shift in policy during an online question and answer session with the public on Friday afternoon.

He said: “I do think we need to be stricter in insisting people wear face coverings in confined spaces where they are meeting people they don’t normally meet.

“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.”

Eminent epidemiologist Professor David Heymann, of the London School of Hygiene & Tropical Medicine, said masks should be worn by “all people in a situation where no one can physically distance to prevent infection of others”.

Face masks do not substitute for physical distancing if physical distancing is possible.

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 PM was “right” to be reviewing England’s position on face coverings.

He said it was unlikely that the scientific debate on the issue would be resolved anytime soon, but that shops may be an example of a place where it is not possible to maintain social distancing.

“However, if it is mandated to wear them in shops, this raises the issue of whether they should be mandated in other contexts,” he explained.

Dr Hunter also cautioned: “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.”

University of Bristol Professor Adam Finn said that “wearing face coverings in crowded places will reduce the likelihood of that happening”.

“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.”

Shadow health secretary Jonathan Ashworth urged the Government to “conclude this review rapidly to provide the strong and clear guidance needed”.

However, UCL epidemiologist Dr Antonio Lazzarino said he was worried masks are “a pretence to ease the lockdown to help the economy”.

“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.”

The latest figures from the Office for National Statistics suggested more than half (52%) of adults in Britain who left their home in the week before being surveyed had worn a face covering.

The survey was conducted between July 2-5 and is up from the previous week, when 43% reported doing so.

Tory former health secretary Jeremy Hunt said people should wear face coverings in shops, as he called for simple Government messaging.

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.”

Earlier this week, the World Health Organisation acknowledged there is “emerging evidence” that Covid-19 could be spread through particles in the air.

And the president of the Royal Society, Professor Venki Ramakrishnan, said everyone should wear a face covering in public to reduce the risk of a second wave of Covid-19 infection.