Boris Johnson has said members of the public should not report neighbours who breach the "rule of six" unless they are having "Animal House" parties with "hot tubs and so forth".
Gatherings of more than six people are now against the law in England as part of measures to the limit the spread of coronavirus.
Those who ignore the new measure face fines of up to £3,200, which applies to both indoor and outdoor settings.
She even suggested families who stopped to talk in the street could be breaking the new laws.
Speaking in an interview with The Sun on Thursday, Mr Johnson has urged people to speak with rule breakers before notifying the authorities.
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He said: "I have never much been in favour of sneak culture, myself.
"What people should do in the first instance is obviously if they are concerned is raise it with their friends and neighbours.
"But I think what is reasonable for anyone to do is if they think there is a serious threat to public health as a result of their neighbours' activities – if there is some huge kind of Animal House party taking place, as I am sure, hot tubs and so forth, and there is a serious threat to public health then it's reasonable for the authorities to know."