The World Health Organization (WHO) has changed its advice on face masks, saying they should be worn in public where social distancing is not possible to help stop the spread of coronavirus.
The global body said new information showed they could provide "a barrier for potentially infectious droplets".
Some countries already recommend or mandate face coverings in public.
The WHO had previously argued there was not enough evidence to say that healthy people should wear masks.
However, WHO director-general Dr Tedros Adhanom Ghebreyesus said on Friday that "in light of evolving evidence, the WHO advises that governments should encourage the general public to wear masks where there is widespread transmission and physical distancing is difficult, such as on public transport, in shops or in other confined or crowded environments".
The organisation had always advised that medical face masks should be worn by people who are sick and by those caring for them.
Globally, there have been 6.7 million confirmed coronavirus cases and nearly 400,000 deaths since the outbreak began late last year, according to data compiled by Johns Hopkins University.
The organisation said its new guidance had been prompted by studies over recent weeks.
Dr Maria Van Kerkhove, the WHO's technical lead expert on Covid-19, told Reuters news agency the recommendation was for people to wear a "fabric mask - that is, a non-medical mask".
Fabric masks should consist of "at least three layers of different material" in order to be effective, the WHO says.
However, those aged over-60 and with underlying health risks should wear medical masks in areas where there is community transmission.
At the same time, the WHO stressed that face masks were just one of a range of tools that could be used to reduce the risk of transmission - and that they should not give people a false sense of protection.
"Masks on their own will not protect you from Covid-19," Dr Tedros said.