Police in London will only enforce the wearing of masks in shops "as a last resort", the Met Commissioner has said.
Dame Cressida Dick said she hoped shoppers who refused to wear masks would be "shamed" into compliance.
Officers will attend if people not wearing a mask refuse to leave a shop or become "aggressive," Dame Cressida said.
Thames Valley and Devon and Cornwall police forces say officers will attend incidents only if they turn violent.
But Dame Cressida said if shop keepers were concerned and had "tried everything else", her officers would try to assist.
She told LBC Radio: "Calling the police should be a last resort for dealing with a mask issue. But of course the law is the law.
"My hope is that the vast majority of people will comply and that people who are not complying will be shamed into complying or shamed to leave the store by the store keepers or by other members of the public.
"If somebody is concerned about what is going on in their store, yes, of course they should call the police and we will try to assist."
Wearing a face covering in shops and supermarkets in England is to become mandatory from 24 July.
Those who fail to comply with the new rules will face a fine of up to £100, the government has announced.
Dame Cressida said supermarkets had managed to maintain social distancing and queuing themselves and had only rarely needed to call the police.
The move will bring England into line with Scotland and nations such as Spain, Italy and Germany.
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.
Mask-wearing has been compulsory on public transport in England and at NHS facilities across the UK since 15 June.