People face fines of up to £3,200 if they do not abide by the new measure, which applies to both indoor and outdoor settings and follows a rapid increase in the number of daily positive cases.
Regulations enabling the enforcement of the rule were published late on Sunday night, around 30 minutes before they came into effect.
More than 3,000 Covid-19 cases were recorded in the UK for the third day in a row on Sunday – the first time since May that cases were above 3,000 on three consecutive days.
The new law comes amid concerns about an increase in cases in care homes and growing criticism of the NHS Test and Trace system.
Meanwhile, Israel will enter a three-week nationwide lockdown starting on Friday to contain the spread of the coronavirus after a second- wave surge of new cases.
During the lockdown, which comes during the Jewish high-holiday season, Israelis will have to stay within 500 metres of their houses, but can travel to workplaces that will be allowed to operate on a limited basis.
Schools and shopping malls will be closed but supermarkets and pharmacies will remain open.
Prime Minister Benjamin Netanyahu said on Sunday: “I know those measures will exact a heavy price on us all, this is not the kind of holiday we are used to. And we certainly won’t be able to celebrate with our extended families.”
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10am update: Row over Northern Ireland COVID-19 restrictions
Stormont Health Minister Robin Swann has dismissed “bogus” claims that he was overruled during last week’s Executive meeting on COVID-19 restrictions.
Senior Democratic Unionist Sammy Wilson said his party leader, First Minister Arlene Foster, had reined in health department “technocrats” who wanted to take more severe measures.
On Thursday, ministers resolved to impose localised limitations on people socialising in Belfast, Ballymena and other areas with high infection rates.
Mr Swann said: “In light to what is being alleged this morning, I am not going to breach the confidentiality of Executive discussions to comment on ill-informed and bogus claims.
“Our public health messaging is too important to play games with.”
Mr Wilson alleged that Mr Swann and others had pressed for more extensive restrictions during Thursday’s Executive meeting.
9:15am update: New restrictions on face coverings for Wales
Face coverings are now mandatory in shops and indoor public spaces in Wales, where it is now illegal for more than six people to meet inside.
First Minister Mark Drakeford has warned that COVID-19 is “on the rise again” in Wales after weeks of being in decline, with 20 people in 100,000 in the country having the virus.
He previously said the new measures, which apply from Monday, were needed to prevent cases from spreading and to avoid more “draconian” restrictions.
8:30am update: Minister tells people to grass up their neighbours if they break COVID rules
Policing minister Kit Malthouse said concerned neighbours should ring the non-emergency police phone number to report violations of the rule of six.
He told BBC Radio 4’s Today programme: “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.
“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.”
7:45am update: Mass redundancies feared at end of Furlough scheme, Union warns
A trade union boss has warned that the end of the job retention scheme could trigger mass redundancies.
Speaking on BBC Breakfast, TUC general secretary Frances O’Grady said: “Unemployment isn’t just miserable for families, it will hold back the economy, just at the time when if we are facing further spikes and lockdowns we need more support.
“We’ve put forward proposals to the Government for a job retention and upskilling scheme and I’m really appealing to the Chancellor to stand by working families and keep people at work.”
She described the furlough scheme as a success, stating it is “too soon to pull the plug”.
“The problem is if the Government doesn’t act now we could be facing mass unemployment and we know how much harm that does to the economy and how much more expensive it is in the long run.”
