There could be times when there is "logic" for wearing a mask outdoors, the British government's top medic has suggested.
Chief medical officer Professor Chris Whitty said there is a "risk" of Covid-19 in "crowded" outdoor environments such as a "queue".
Prof Whitty told BBC Radio 4's Today programme the risks from, for example, jogging past someone were "very low", but there could be an argument for wearing masks in other circumstances.
"If people for example are crowded together in a queue outdoors, if they're really huddled together round a market stall or something - that is a risk with this virus - and in that situation there might be some logic to people thinking about wearing masks."
Pressed on whether people should wear masks in all outdoor settings, he said the most important thing was actually that people stayed home unless their journey was essential.
"I think that the much more important thing is that people should not be leaving their home unless they absolutely have to," he said.
"And where they do, try and keep their distance from people."
Other scientists agreed that scenarios such as queues or where air is "stagnant" could pose a risk.
Dr Shaun Fitzgerald, a visiting professor at the University of Cambridge, said the "risk outdoors is much lower than indoors" but that "the risk will be higher especially if you are stationary".
He added: "If you are queuing at a market stall which is only really open to one side then this is very different from passing someone on a walk by the sea.
"I absolutely agree with the suggestion of wearing face coverings outdoors in environments where the air is relatively stagnant and where queues form."
An infectious disease expert reiterated the importance of two-metre distancing especially at supermarkets.
Prof Keith Neal, from the University of Nottingham, said: "Standing in queues, such as for supermarkets, for prolonged periods with less than two metres distance puts people at a substantially higher risk of spreading or catching the virus than simply passing on the pavement.
"In these circumstances wearing masks is prudent but more distancing is required.
"Better still would be two-metre distance markers for the queue as in the supermarket I do my shopping in.
"There is no need to stand close to people, you will get into the store."
