In England, people who see their neighbours flouting the new “rule of six” are being encouraged to contact police by the government.
Social gatherings of more than six people are now illegal in England from today.
People face fines of up to £3,200 if they do not abide by the new measure, which aims to curb an increase in COVID-19 cases.
A total of 3,330 confirmed coronavirus cases were recorded in the UK on Sunday – the first time since May that cases have been above 3,000 on three consecutive days.
As the new rule came into force, policing minister Kit Malthouse said concerned neighbours should ring the non-emergency police phone number to report violations of the restriction.
“We are in discussions about what reporting mechanisms there might be but there is obviously the non-emergency number that people can ring and report issues they wish to,” he told the BBC Radio 4’s Today programme.
“Certainly during the initial stage of lockdown, we did see a surge in those reports coming through to the police.
“If people are concerned, if they do think there is contravention then that option is open to them.”
Pressed on whether that would involve reporting a gathering of seven or more in a neighbour’s garden, Mr Malthouse said: “It is open to neighbours to do exactly that through the non-emergency number and if they are concerned and they do see that kind of thing, then absolutely they should think about it.”
Asked to clarify whether residents should be concerned about local breaches of law in neighbouring properties and report them to the police, he replied: “Yes.”
Regulations enabling the enforcement of the rule were published late on Sunday night, around 30 minutes before they came into effect.
Home Secretary Priti Patel said the new rule was “easier to understand and easier for the police to enforce”.
Writing in The Sun newspaper, Ms Patel said the police “will be able to disperse gatherings of more than six people and fine individuals involved £100, doubling up to a maximum of £3,200 for repeat offenders”.
“They will continue to hand down £10,000 fines for people hosting raves or other large unlawful events,” she added.
“I know that, as part of our national effort, the law-abiding majority will stick to these new rules.
“But there will be a small minority who do not and the police have the necessary powers to take action against them.
“This disease is deadly and that is why it is right that the police enforce where people break the rules.”
Downing Street suggested police would not immediately start imposing fines on people who break the “rule of six” restrictions.
The Prime Minister’s official spokesman said: “What you would expect to happen is for the police to be out today encouraging people to follow the new rules but in the coming days, if we see people continuing to flout the new rules, it is right that people could face a fine.
“The regulations are in place to help to stop the spread of the virus, to protect the NHS and to ultimately save lives.”
Asked whether people should report neighbours who breach coronavirus rules, the spokesman said: “What we want people to be focused on is following the new rules and ensuring that they, themselves, help again as they have in the past to slow the spread of the virus.
“What you have seen in recent weeks is some egregious flouting of the rules, such as the holding of large illegal parties, and members of the public have been contacting the police about those because they have been concerned about the risk to public health.”