LONDON — The British government performed an about-face on masks Monday, telling people to cover their mouth and nose in shops, buses and subway trains. The change came as part of what Prime Minister Boris Johnson called the ”first careful measures” to lift a nationwide lockdown imposed seven weeks ago to slow the spread of the new coronavirus.

A 50-page government document outlining cautious steps to ease restrictions said “people should aim to wear a face-covering in enclosed spaces where social distancing is not always possible and they come into contact with others that they do not normally meet, for example on public transport or in some shops.”

That is a recommendation rather than a rule, and people won’t be penalized if they don’t wear a mask.

The government outlined a three-stage approach to ending lockdown, beginning Wednesday with a relaxing of limits on outdoor activity. People in England may take unlimited amounts of exercise, rather than just one trip out a day, and may sit and sunbathe outdoors. Driving to a park or beach will be permitted, and golf courses and tennis courts can reopen.

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If there is no new spike in infections,

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