Major supermarkets and coffee chains say they will not enforce new rules which say customers should wear face coverings from Friday.
Sainsburys, Asda, Co-op and Costa Coffee are among retailers saying they have no intention of policing the laws, which carry a penalty of a £100 fine.
It comes after the Police Federation of England and Wales, which represents rank-and-file officers, said it was "unrealistic and unfair" to expect them to patrol the aisles looking for people breaking the coronavirus regulations.
And Metropolitan Police Commissioner Dame Cressida Dick said she hoped shoppers who refuse to wear masks would be "shamed" into compliance.
After weeks of confusion, the Government on Thursday issued guidance, setting out the situations and venues in which face coverings must be worn.
From Friday, customers in England will be breaking coronavirus laws if they do not wear a face covering in shops, shopping centres, banks, takeaway outlets, post offices, sandwich shops and supermarkets (as outlined below). This includes when buying food and drink to take away from cafes and shops, like Pret A Manger.
However, it is not compulsory for shop or supermarket staff to wear face coverings, although the guidance “strongly recommends” their use.
The new advice sets out situations in which shoppers will be expected to wear coverings - but to be ready to remove them if challenged. Those buying alcohol, or visiting a bank or post office, could be asked to show their full face, in order to prove identity, or age.
With police reluctant to enforce the new laws, the Government is forced to enlist the help of retailers in ensuring the changes are made.
But on Thursday several said they had no intention of challenging customers who enter their stores unmasked.
The Government guidance makes clear that the responsibility for wearing a face covering "sits with individuals", not people who work in shops or cafes.
But it says: "Businesses are encouraged to take reasonable steps to encourage customers to follow the law, including through signs and providing other information in store."
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Sainsbury's said while it is asking everyone to continue "playing their part" in helping to keep everyone safe in store by following the rules, "our colleagues will not be responsible for enforcing them".
Asda said it will "strongly encourage customers to wear a face covering", but added: "It is the responsibility of the relevant authorities to police and enforce the new rules."
Costa Coffee said it would "not be challenging customers who enter our stores without a mask since they may have a legitimate reason as to why they are unable to wear one."
Paul Gerrard, director of public affairs at Co-op Food, said shop workers should not be expected to enforce rules on face coverings, saying: "It's not their job, it's the police's job".
Tesco said it would be selling face coverings at the entrance and Waitrose said staff would be at the entrance to stores reminding customers of the requirement.
The Association of Convenience Stores has already said: “We have advised members not to challenge customers unwilling to wear a covering.”
Customers will be "required" to wear a face covering in Greggs, the bakers. And fast food chain McDonald's said takeaway customers will need to wear face coverings but those who eat in the restaurant will not unless they are moving around the premises.
The Government guidance says coverings must be worn in places including shops, supermarkets, banks, building societies and post offices. Exemptions include visitors to museums, cinemas, concert halls, theatres, hairdressers, gyms and leisure centres, dentists and opticians.
Anyone who fails to wear a face mask - defined by the Government as a "fabric covering, scarf or bandana" - covering their nose and mouth will be risking a £100 fine from the police, the Government said.
Matt Hancock, Health and Care Secretary, said: “As we move into the next stage of easing restrictions for the public, it is vital we continue to shop safely so that we can make the most of our fantastic retail industry this summer.
“Everyone must play their part in fighting this virus by following this new guidance.”
The guidance states that the role of the police is to “engage, explain, encourage and finally enforce as a last resort”.
The British Medical Association (BMA) on Thursday night questioned why it took so long for mandatory face coverings to be introduced, and criticised the Government for only issuing guidance on the matter the day before the rules come in.
Dr Chaand Nagpaul, BMA council chairman, said the measures were “long overdue” but said weeks of uncertainty had “done nothing to inspire public confidence”.
Police and crime commissioners for Devon and Cornwall and Thames Valley have said officers will not attend incidents where shoppers refuse to wear masks, unless they turn violent.
Andy Marsh, chief constable of Avon and Somerset Police, said the force had "very limited capacity" to enforce the rules, and would only have a role in "a tiny minority" of cases.
The Prime Minister's official spokesman said on Thursday: "With shops, we would expect them to give advice to customers and remind them that they should be wearing a face covering and I'm sure the overwhelming majority of the public will do so."
Children under 11 are exempt, as well as those with disabilities, or a range of health conditions including breathing difficulties and other respiratory conditions, dementia and autism.
Latest data from the Office for National Statistics shows 71 per cent of adults are now wearing a covering when they leave home, up from 61 per cent in a week.
Public Health England (PHE) on Thursday warned parents not to buy face coverings for babies and young children, after health officials found retailers targeting the young. They warned that children under the age of three are at risk of choking and suffocating if their faces are covered.
It came as scientists set out advice that home-made face coverings should be at least two layers thick - with three layers better - for preventing the spread of droplets from speaking, coughing and sneezing.
Although Department of Health guidance states that wearing a scarf is sufficient, a new study in the BMJ Thorax journal study suggests that the fabric would need to be folded several times for full protection.
Experts used a high-speed to capture the light reflected by droplets and aerosols expelled during speaking, coughing and sneezing, while a volunteer wore different types of mask.
They found that while one layer of cotton T-shirt material is “fairly” effective as a barrier against droplets expelled during speaking, two are “significantly better at reducing the droplet spread caused by coughing and sneezing”.
Three layers are even better, the researchers from the University of New South Wales in Australia said, while surgical masks are best of all.