All shoppers must wear a face mask from today, but police enforcement of these rules would be a "last resort".

Hertfordshire Constabulary said enforcement of the new law should be a joint effort with shop staff and police will only get involved after "all efforts from retail staff have been exhausted".

This comes as several big supermarkets - including Sainsbury's, Tesco, Waitrose and Lidl - and Costa Coffee confirmed they would not refuse entry to those without a face mask, nor would shop staff challenge any non-mask-wearing customers.

Three days ago, Hertfordshire Constabulary said it did not yet know how it would enforce the new law and said it was still waiting for formal guidance.

When the Observer asked again today (July 24) how it would enforce the rules, a spokesperson said: "The vast majority of people are complying with the regulations to manage the spread of Covid-19 and hopefully this will continue as a joint effort with retailers and their staff encouraging customers to wear face coverings inside their premises.

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"Police involvement will be a last resort for situations where all efforts from retail staff have been exhausted, customers refuse to leave or staff are facing aggression.

"Staff working in shops and other enclosed places included in the regulations will be working hard to keep people safe in their premises. We will take very seriously any abuse or violence used against them for simply doing their jobs.

"As with other coronavirus regulations, Hertfordshire Constabulary will follow an approach of engaging, explaining, encouraging and only enforcing where encouragement has been unsuccessful."

The Police Federation of England and Wales is also urging shops to refuse entry to those not wearing face masks without a valid reason.

John Apter, the national chairman of the Police Federation of England and Wales, said forces “do not have the resources” to widely enforce the law that came into force on Friday.

Borehamwood Times:

Face coverings are are mandatory in shops and other settings in England from today (July 24). Photo: Andrew Milligan/PA

He said: “It is our members who are expected to police what is a new way of living and I would urge retail outlets to play their part in making the rules crystal clear – if you are not wearing a face covering then you are not coming in.

“Officers will be there to help stores if needed – but only as a last resort, as we simply do not have the resources."

Guidance issued by the Government on Thursday states that staff in premises where face coverings are required are encouraged to “take reasonable steps to promote compliance with the law” and can refuse entry to people who do not have a valid exemption under the rules.

Tom Ironside, director of business and regulation at the British Retail Consortium trade association, said the role of retailers is to “remind and encourage compliance”.

Borehamwood Times:

People who do not obey the rules could be fined £100. Photo: PA

“Where the rules aren’t being followed enforcement will be a matter for police,” he said.

“We expect and hope this will only apply in a small minority of cases.

“What’s very clear is enforcement is not a matter for retailers, that has to be a matter for the police because otherwise you will be asking staff in stores to do things which can add to the potential for conflict.”

Retail union leaders have also voiced fears the rules could put workers’ safety at risk if there are abusive customers or people who refuse to wear a mask.

The laws could be in place until at least January, and even last a year, unless the Government decides to scrap them.