British Prime Minister Boris Johnson has urged people to limit their social contact "as much as possible" and to minimise interactions with other households as he outlined new measures to control a surge in coronavirus cases.
Mr Johnson said he was "sorry" that two households that exceed six people would no longer be able to meet in England from Monday, and said he wished that he did not have to take such steps.
But addressing the first Downing Street press conference since July, Mr Johnson said that "if we are to beat the virus then everyone, at all times, should limit social contact as much as possible".
"It is safer to meet outdoors and you should keep your distance from anyone you don't live with, even if they are close friends or family.
"So in England, from Monday, we are introducing the rule of six. You must not meet socially in groups of more than six - and if you do, you will be breaking the law.
"This will apply in any setting, indoors or outdoors, at home or in the pub.
"The ban will be set out in law and it will be enforced by the police - anyone breaking the rules risks being dispersed, fined and possibly arrested."
Other measures announced today include a legal requirement for venues where people meet socially, such as pubs and restaurants, to request contact details of every member of a party and retain the information for 21 days. Fines of £1,000 could be levied against hospitality venues if they fail to comply.
Passengers travelling to the UK will need to fill out a simplified form with their contact details before they depart, while the Border Force will step up enforcement efforts to ensure compliance with quarantine rules.
Plans to pilot larger audiences in venues later this month will be revised, and the government is reviewing its intention to return audiences to stadiums and conference centres from 1 October.
The opening hours of some venues could be restricted in some local areas. It comes after hospitality venues in Bolton were required to close between 10pm and 5am.
"Covid-secure marshals" will be introduced to help ensure social distancing in town and city centres in a bid to improve the enforcement capacity of local authorities.
The new rules follow a rise in cases from 12.5 per 100,000 people to 19.7 per 100,000 in the UK in the last week - with a particular rise in infections among young people.
Infections are most prevalent among the 19 to 21-year-old age group, with 54 cases per 100,000 people.
The uptick suggests the national R number is now above 1 - meaning the virus is spreading exponentially.
Mr Johnson also urged people not to book a coronavirus test unless they have symptoms or have been asked to book a test, following reports that people were being denied Covid-19 tests.
"We are increasing our testing capacity further to meet rising demand. You can help by only booking a test if you have a fever, a new continuous cough, or you've lost your sense of taste or smell - if you don't have those symptoms and haven't been asked to book a test, please don't," he said.
Some 2,420 lab-confirmed cases of coronavirus were recorded in Britain as of 9am yesterday, following the 2,988 reported in the UK on Sunday, which was the largest daily figure since May.
Number 10 said Mr Johnson held a virtual roundtable with police forces last week where officers expressed their desire for rules on social contact to be simplified.
But the Police Federation of England and Wales urged the government to "play its part" through a public information campaign after "so many changes in legislation".
The association's national chairman John Apter said: "For policing, these constant changes to legislation are becoming the norm. The pressures on policing have increased significantly over recent months and this latest change will add to this pressure.
"My colleagues will support the public through what is going to be a very difficult time. At all times they will also remind people that a breach of these regulations means breaking the law.
"However, the Government needs to play its part. With so many changes in legislation, an effective public information campaign must be a priority, as there's been so much confusion for the public and many people don't know exactly what the law says."
Different rules apply to social gatherings elsewhere in the UK.
In Scotland, up to eight people are allowed to meet indoors, while groups of 15 from up to five households are permitted outdoors, but people should maintain a social distance.
People there should not meet people from more than four other households in total whether indoors and/or outdoors.
In Wales, up to 30 people can meet outdoors and in Northern Ireland the number is 15.
The change to the law comes after Mr Johnson told his cabinet that ministers must ensure there is "no complacency" among the public, and particularly young people, following the rise in coronavirus infections.
Health Secretary Matt Hancock warned of a possible second peak, as he imposed further measures in Bolton restricting all hospitality to takeaways only, as well as new rules meaning venues will have to close between 10pm and 5am.
Meanwhile, Welsh Health Minister Vaughan Gething said a local coronavirus lockdown is being imposed in Caerphilly because there is evidence of "community transmission" in the area.
People will not be allowed to enter or leave the area without a reasonable excuse after the restrictions came into force at 6pm yesterday.
