Face masks will become mandatory in shops across England from 24 July the government has announced.
Those who fail to comply with the new rules will be fined up to £100 for non-compliance.
It comes more than a week after Nicola Sturgeon, the Scottish first minister, announced face coverings would be compulsory in shops in Scotland, which came into force on Friday. In England, masks will be mandatory in supermarkets and all other shops, with other locations kept under review.
Health Secretary Matt Hancock said it would "give people more confidence to shop safely and enhance protections for those who work in shops".
Since mid-May, members of the public have been advised to wear coverings in enclosed public spaces, where they may encounter people they would not usually meet.
Johnson himself wore a mask for the first time in public last week, and went further on Monday, urging the public in England to wear masks in shops as “extra insurance” against the coronavirus.
The government had come under increased public pressure to go further on face coverings amid mounting evidence that masks can effectively hinder transmission of the virus. They were made mandatory on public transport in England on 15 June.
Children under 11 and those with certain disabilities will be exempt.
The list of exemptions has not yet been published, but the rules for face masks on public transport exempt anyone who cannot wear one "because of any physical or mental illness or impairment, or disability", who would experience "severe distress" from doing so, or relies on lip reading, among other reasons.
The latest rules for shops will be enforced by the police, with anyone disregarding them at risk of a fine of up to £100. This will be reduced to £50 if the fine is paid within 14 days.
"A shop can refuse them entry and can call the police if they refuse to comply," Mr Hancock said.
Martin Hewitt, chair of the National Police Chiefs' Council, said police should be involved only as a "last resort".
Downing Street said emerging evidence highlighted by the World Health Organization on airborne transmission of the virus is one of the key factors behind the changing advice.
The Independent Sage group, led by the government’s former chief scientific adviser, Sir David King, said evidence had been growing for legislation to mandate mask-wearing. However, the group said it was “essential that masks are not seen as a "panacea and other measures such as rigorous hand washing and social distancing are still stringently followed”.
The UK have been among the slowest to embrace mass mask-wearing. Many European countries, including Germany, Spain, Italy and Greece, have already made it compulsory to wear face coverings inside shops.