As the PM says Britons should be wearing face coverings in shops, we look at whether they are effective against COVID-19.
People must wear face coverings in shops in England from July 24, the government has announced.
But until now scientists have criticised ministers for their mixed messaging about wearing face masks during the pandemic, so what does the science actually say?
Meanwhile, cabinet minister Robert Buckland claimed "people are still learning how to use face coverings" after Home Secretary Priti Patel was pictured wearing a mask when meeting her French counterpart outside - but not wearing one when they spoke indoors.
The official UK government guidance states that evidence around wearing a face covering suggests it "does not protect you" from coronavirus.
But the guidance adds: "If you are infected but have not yet developed symptoms, it may provide some protection for others you come into close contact with."
The evidence of coverings preventing the spread of infection from one person to another is "marginal but positive", according to the Scientific Advisory Group for Emergencies that is advising the government.
But a study by Cambridge University says even basic homemade masks can reduce transmission - and could even help to prevent a second wave.
It said population-wide use of masks would keep the COVID-19 reproduction number (R rate) below one.
An international report published in The Lancet, which analysed data from 172 studies in 16 countries, found that by wearing a face mask there is just a 3% chance of catching COVID-19.
Another study found that homemade face masks can help limit the spread of the coronavirus.
Seven types of face masks were put to the test by the University of Edinburgh, including surgical masks, respirators, lightweight and heavy-duty face shields, and handmade masks.
Aside from those with a valve, all of the face coverings were found to reduce the forward distance travelled by an exhaled breath by at least 90%.
However the World Health Organisation (WHO) has stressed that masks "on their own" will not protect from COVID-19.
It says there is no evidence that wearing one - whether medical or other types - by healthy persons in the wider community can prevent them from being infected with respiratory viruses, including the coronavirus.
Fears have also been raised that they could give people a false sense of security and mean they are less observant of the rules around social distancing and hand hygiene.
The government's official advice states that surgical masks should be reserved for people who need them for protection while at work.
It encourages people to make face coverings at home, saying "the key thing is it should cover your mouth and nose".
source : news.sky.com