Boris Johnson has been criticised on social media for apparently failing to remove his face mask properly.
Twitter users said the prime minister had demonstrated how not to take a mask off.
Face coverings in shops and supermarkets in England will be mandatory from 24 July.
Monday’s announcement came after government was accused of sending mixed messages over the issue at the weekend.
The PM had said people should wear them in shops but cabinet minister Michael Gove insisted they shouldn’t be compulsory.
Even early on Monday, Downing Street said a decision would be taken in “the next few days”, but by the evening it emerged that masks will be mandatory in shops in England from 24 July.
Johnson was seen in a face mask in public for the first time last Friday while inside a shop in his Uxbridge constituency, and wore one again on Monday during a visit to the London Ambulance Service.
However, Johnson has come under scrutiny for apparently failing to remove his mask properly during the London Ambulance visit.
A one-second video posted on Twitter by Daily Mail deputy political editor John Stevens showed Johnson taking his mask off in front of an ambulance.
Stevens captioned the video: “A guide on how not to take a face mask off.”
In the quick clip, Johnson removes his covering by pulling the front of the mask from his face, then rolling it up in his hands.
Twitter users were quick to point out the official guidance, from both his own government and the World Health Organization (WHO), on the correct way to remove a face mask.
The government’s own instructions, from Public Health England, advise people to wash their hands and avoid touching the front of the mask, using the ear loops instead.
It says: “Wash your hands or use hand sanitiser before putting it on and after taking it off.
“Avoid touching your eyes, nose, or mouth at all times and store used face coverings in a plastic bag until you have an opportunity to wash them.
“Do not touch the front of the face covering, or the part of the face covering that has been in contact with your mouth and nose. Once removed, make sure you clean any surfaces the face covering has touched.
“You should wash a face covering regularly. It can go in with other laundry, using your normal detergent.”
In its guidance on face masks, the WHO says: “While in use, avoid touching the mask.
“Remove the mask by using appropriate technique (i.e. do not touch the front but remove the lace from behind). After removal or whenever you inadvertently touch a used mask, clean hands by using an alcohol-based hand rub or soap and water if visibly soiled.”
The WHO says users should wash their hands before putting on a mask and wash them again after removing it and throwing it in a closed bin.
Downing Street has refused to comment on how the prime minister removed his mask.
On Tuesday, health secretary Matt Hancock will say anyone failing to wear a face mask in shops and supermarkets in England from 24 July could face a fine of up to £100 – reduced to £50 if paid within 14 days.
Making masks mandatory will bring England into line with Scotland, where face coverings are already compulsory in shops.
The government has been urging people to wear face coverings in confined spaces such as shops since early May and they have already been made compulsory on public transport in England since the middle of June.
Enforcement of the regulations will be the responsibility of the police. While shop workers will be asked to encourage compliance, retailers and businesses will not be expected to enforce them.
But one police boss said the move will be “impossible to enforce”.
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