Enforcing the wearing of face coverings in shops will be "impossible", the chairman of the Police Federation has told Sky News as legislation comes into force in England.
John Apter, who represents rank and file officers in England and Wales, said forces simply do not have the manpower to enforce the new measure.
He said: "Not only is it going to be difficult, it is going to be impossible. We simply can't do it.
"Policing has been stretched like never before. There are not enough of us to make this work."
Boris Johnson said: "I think we should rely on the massive common sense of the British people that have so far delivered the results that we've seen, and that's going to work."
People understand the value of face masks in confined places, the prime minister added.
Those who fail to wear a face covering will face a fine of up to £100 - in line with those failing to wear a face mask on public transport.
The fine will reduce to £50 if paid within 14 days.
Mr Apter said officers will only attend shops or supermarkets if people not wearing a face covering refuse to leave a shop.
"If people think they are going to see police officers outside every single store to ensure people are wearing masks then I am afraid they are going to be disappointed."
Police say it is the responsibility of shop managers to ask non-mask wearers to leave their stores instead of calling them out to solve the problem.
"If people fail to comply with it, the police can be called in to deal with it," Mr Apter said.
"I would hope what we will see is what we have seen throughout this whole pandemic and that is the vast majority of the public complying."
According to the College of Policing, officers will have the power to "use force" to remove customers from shops if they do not wear face coverings, as well as prevent them from entering.
The new law does not apply to staff working in shops and people are not required to wear masks inside pubs, bars or restaurants.
It does include banks and supermarkets.
In Scotland, it is required by law to wear face coverings in both shops and on public transport.
People are not currently required to wear them in Wales or Northern Ireland.
Shop owner Diane Bardon accused the government of "mixed messaging" and said she won't enforce restrictions on customers who refuse to cover up.
She told Sky News: "I am not going to call the police on my customers. If I give them a mask and they don't want to wear it then so be it. I can't force them to do it. I would allow them. This is my livelihood. Who is going to pay my rent and bills if I don't?"
Some people are exempt from the rules - children under 11 and those with breathing problems are not required to wear a covering.
The government has been accused of being unclear in its own advice over face coverings, leading to confusion as to where people should be wearing them.