Boris Johnson has said people in England “should be wearing” face masks or other coverings inside shops to help prevent the spread of coronavirus.
The prime minister said the government would decide in the next few days if “tools of enforcement” were needed.
The comments follow cabinet minister Michael Gove telling the BBC on Sunday that face coverings should not become mandatory in shops in England.
Labour has demanded “urgent clarity” from the government on the issue.
And the boss of Waterstones bookstores, James Daunt, said it “would not be right” to ask shop workers to “police” any new policy.
Face coverings are worn to help prevent wearers spreading coronavirus, rather than catching it.
Currently, they are compulsory on public transport in England, Scotland and Northern Ireland – and the Welsh government has announced it will enforce the same measure from 27 July.
In Scotland, coverings are also mandatory in shops, but not elsewhere in the UK – and critics have complained that the situation in England needs to be made easier for the public to understand.
Analysis by Helen Catt, political correspondent
The signs seem to point towards the government making face masks compulsory in more places in England, but ministers appear reluctant to commit just yet.
Michael Gove’s preference for trying to encourage people to take action voluntarily – rather than through fear of enforcement – is one we’ve seen the government repeat throughout the pandemic.
But the risk that comes with it is of mixed messaging. Earlier, when coronavirus was more widespread, it said the scientific evidence for mask-wearing was not clear enough. Now, it says, the evidence is stronger.
Of course, the scientific understanding of the virus is constantly developing, and so policy is likely to as well.
If the government does now think masks are the way forward though, communicating that message without confusion is going to be key.
The World Health Organization says masks or homemade cloth face coverings should be worn in public where social distancing is not possible to reduce the spread of coronavirus droplets.
It changed its advice last month, having previously argued there was not enough scientific evidence to say that healthy people should use them.
Visiting ambulance staff in central London, Mr Johnson, whose government controls health policy in England but not the rest of the UK, said: “I think people should be wearing [face masks] in shops.
“And, in terms of how we do that whether we make it mandatory or not, we’ll be looking at the guidance – we’ll be seeing a little bit more in the next few days.”
Mr Johnson added: “Throughout this crisis people have shown amazing sensitivity towards other people and understanding of the needs to get the virus down by doing things cooperatively.
“Wearing masks is one of them… It’s a mutual thing; people do see the value of it. We’ll be looking in the next few days about exactly how – with what tools of enforcement – we think we want to make progress.”
The statement follows some confusion over the government’s intentions in recent days.
The prime minister said on Friday: “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.”
But on Sunday, Cabinet Office minister Mr Gove told the BBC’s Andrew Marr Show he did not think wearing masks in shops in England should be compulsory, adding that he would “encourage” them to do so “where they are likely to be mixing with others and where the ventilation may not be as good as it might”.
He added: “I think that it is basic good manners, courtesy and consideration, to wear a face mask if you are, for example, in a shop.”
Labour’s shadow health secretary Jonathan Ashworth said that “conflicting advice and conflicting statements from the government only hinder our fight against the virus”.
He has written to Health Secretary Matt Hancock, asking him to “urgently set out the position on face coverings”.
“As lockdown rules are further relaxed this week, it is vital that updated guidance on this issue is published by the government without delay,” Mr Ashworth added.
Paddy Lillis, general secretary of the shop workers’ union Usdaw, said “mixed messaging” on face coverings was “not helpful” for staff.