MPs have called for the government to clarify its position on face coverings, after the PM said a "stricter" approach to their use in England was needed.
Senior sources have said the government is considering making face coverings mandatory in shops in England to slow the spread of coronavirus.
Former Health Secretary Jeremy Hunt said he was "in favour" of the change, adding a simple public message was key.
Labour MP Jonathan Ashworth said "strong and clear guidance" was needed.
A further 148 deaths were recorded in the UK on Friday, bringing the total number of recorded deaths to 44,798.
Face coverings have been compulsory in shops in Scotland since Friday.
Currently people in England are advised to wear a face covering in enclosed public spaces where social distancing is not possible - on top of the mandatory requirement to wear one on public transport and when visiting hospitals.
Mr Hunt, chairman of the Commons Health Select Committee, said he understood the current advice but it did not answer the "basic question" of whether people needed to wear a face covering when shopping.
He told Radio 4's Today programme: "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."
It came as Boris Johnson was pictured wearing a face covering for the first time while visiting businesses in his Uxbridge constituency on Friday.
The UK government was initially hesitant - in the early days of the coronavirus crisis - about advising people to wear face coverings, arguing the scientific evidence that they reduce transmission was "weak".
However, rules compelling people to wear face masks on public transport in England were introduced on 15 June.
And on Friday Mr Johnson said "the balance of scientific opinion seems to have shifted more in favour of them than it was, and we're very keen to follow that".
He told a Facebook Q&A with the public: "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."
Wales' First Minister Mark Drakeford said he did not think the evidence on the benefits of wearing facemasks was "decisive" yet but said the "balance is shifting".
Speaking to BBC Breakfast, Mr Drakeford said the view of the chief medical officer for Wales was still that face coverings should be recommended but not required. However, he said if the weight of evidence changed "then we will change our policy".
The rules on face coverings vary across the nations. People in Wales are being asked to wear non-medical face coverings where social distancing is not possible. In Northern Ireland, wearing face coverings on public transport became mandatory on 10 July.
Mr Ashworth, the shadow health secretary, said it was "welcome" ministers were looking again at the evidence on face masks.
"We urge ministers to conclude this review rapidly to provide the strong and clear guidance needed," he wrote on Twitter.
Tory MP and former cabinet minister Andrea Leadsom told BBC Radio 4's Any Questions she did not want masks to be mandatory but "people should be considerate of others" and start to think about carrying one wherever they go.
On Friday Boris Johnson urged people to go back to work. He also hinted at a stricter policy on face masks.
Lo and behold he was pictured, out and about at a shop and takeaway, wearing a face mask. It doesn't take a detective to spot an overarching theme.
Ministers want people to start embracing the new normal; leaving their homes, spending money and helping businesses get back on their feet - but yes with mitigation measures, like masks, in place.
Those mitigations are of course there to help suppress transmission but can also play their part in helping people feel confident.
Because public confidence is important; you can urge someone to do something but they may well resist unless they feel safe.
Homemade cloth face coverings can help reduce the spread of coronavirus from people who are contagious but have no symptoms or are yet to develop symptoms.
Professor Trisha Greenhalgh from Oxford University, who argued for widespread use of face coverings in April, told BBC Radio 4's Today programme the vast majority of scientists "are now persuaded by the evidence" that wearing masks in crowded places can reduce transmission.
Asked about the suggestion that face masks benefit others more than the wearer, she said: "Broadly speaking, if I'm wearing a face covering it might protect you 80% and it might protect me 20% or 30%.
"There is a little bit of protection for the person wearing the face coverings but it's not nearly as much as it protects other people from the droplets coming out of the person's mouth."
Prof Greenhalgh added: "Indoor environments are much, much more dangerous in terms of what we call airborne transmission. Someone coughs or speaks loudly and the virus then gets into the air and it stays in that shop or in that indoor space."
The World Health Organization says non-medical face coverings should be worn where social distancing is not possible.
How has the UK government's position on face coverings changed?
- 12 March: England's deputy chief medical officer Dr Jenny Harries says it is "not a good idea" for the average member of the public to wear a face mask as they may not use it properly and put themselves at more risk of contracting coronavirus
- 17 April: The transport secretary says the evidence on whether masks work is "quite mixed" and it is possible wearing them "could do more harm than good in certain situations"
- 1 May: Downing Street says face coverings have a "weak but positive effect" in reducing the spread of coronavirus
- 11 May: People in England are advised to wear face coverings on public transport and in enclosed spaces where social distancing is not always possible
- 15 June: Face coverings are made compulsory on public transport in England
Meanwhile, outdoor pools can now reopen in England and amateur cricket, theatre and other live performances are also allowed outdoors.
In Wales, holiday accommodation without shared facilities is also allowed to reopen from Saturday.
A further 512 coronavirus cases were reported in the UK on Friday, and the Department of Health said 48 more people had died with the virus.
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