While visiting ambulance workers in London, Johnson said: "I think people should be wearing [face coverings] in shops.”
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"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,” he said, according to the BBC.
Johnson’s comments come after critics accused his government of failing to provide clarity on mask-wearing.
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However, cabinet minister Michael Gove on Sunday told the BBC that masks should not be mandatory inside shops, instead said it was "basic good manners, courtesy and consideration" to wear one.
Labour's shadow health secretary Jonathan Ashworth wrote a letter to Health Secretary Matt Hancock, saying that "conflicting advice and conflicting statements from the government only hinder our fight against the virus."
"The confusion around the use of face coverings and whether they will become mandatory needs to be addressed through a statement from ministers as a matter of priority," Ashworth wrote, according to Sky News.
Face masks are already mandatory on public transportation in England, Scotland and Northern Ireland. Some Scottish shops also require masks, according to the outlet. Johnson’s government only controls the health policy in England.
The U.K. had 291,685 confirmed coronavirus cases and 44,915 deaths as of Monday, according to a tally from Johns Hopkins University.
The Associated Press contributed to this report.