Prime Minister Boris Johnson has urged British people to adopt a "rule of six" for social gatherings, saying the new limit was necessary to curb the country's resurgent coronavirus caseload.
Johnson said the government had been forced to cut the maximum size of social groups from 30 to six from Monday, partly because people had failed to follow current rules.
The rules cover all indoor and outdoor settings in England with the exception of schools and colleges, weddings, funerals and team sports.
Devolved governments in Scotland, Wales and Northern Ireland have introduced similar but separate rules.
"It breaks my heart to have to insist on these restrictions upon individuals, upon families, grandparents," Johnson told reporters.
"I certainly don't want to blame people but now is the time for us to focus, to concentrate and to enforce the rule of six."
Johnson urged people to "limit social contact as much as possible" in a bid to reduce the infection rate.
"The ban (on groups of more than six) will be set out in law and it will be enforced by the police," he said.
"Anyone breaking the rules risks being dispersed, fined and possibly arrested."
Britain reported 2460 new COVID-19 infections on Tuesday and 2659 on Wednesday, bringing its total since March to more than 355,000.
The infection rate has jumped by more than 50 per cent to about 20 per 100,000 over the last week, fuelled by more infections among younger people.
Health Secretary Matt Hancock said earlier on Wednesday that the new rules would apply for the "foreseeable future," with the aim of curbing a resurgence in coronavirus infections by the end of the year.
"I really hope we can turn this round before Christmas," Hancock told the BBC.
"Three months is a long time in a pandemic, and I very much hope this strong rule, together with the local action we've taken... can work to do that."
Johnson said it was "too early" to say if families might be asked to have smaller gatherings at Christmas.
He said the government first needed to see the results of its planned "moonshot" programme of mass testing.
People failing to comply with the new measures could face a 100-pound ($A179) fine, doubling on each repeat offence and increasing up to 3200 pounds.
Britain's official death toll linked to the coronavirus pandemic is more than 41,500, Europe's highest total.