Social gatherings of more than six people will be illegal in England from Monday - with some exemptions - amid a steep rise in coronavirus cases.
The law change will ban larger groups meeting anywhere socially indoors or outdoors, the government said.
But it will not apply to schools, workplaces or Covid-secure weddings, funerals and organised team sports.
It will be enforced through a £100 fine if people fail to comply, doubling on each offence up to a maximum of £3,200.
The new rules - which come into force on 14 September - mark a change to England's current guidance.
At present, the guidance says two households of any size are allowed to meet indoors or outdoors, or up to six people from different households outdoors. Until now the police have had no powers to stop gatherings unless they exceeded 30.
Prime Minister Boris Johnson will give further details of the changes at a Downing Street news conference at 16:00 BST on Wednesday, alongside senior advisers Prof Chris Whitty and Sir Patrick Vallance.
"One of the pieces of feedback we had including from the police was that we needed the rules to be super simple so that everybody knows what they are," said Health Secretary Matt Hancock.
"And now this will now be rigorously enforced by the police."
Mr Hancock told BBC Breakfast: "This is really simple. Gatherings are ok, they should be socially distanced of course, but groups only of up to six."
He said it means that some families will not be able to see both grandparents at the same time. "You know, I have three children," he said. "We have a family of five. And so we'll be able to see one other person at a time, as a whole family."
Challenged on whether the new rules could stop grandparents helping out with childcare, Mr Hancock said: "I'm not actively trying to do that".
But he said the government is "quite worried about" the "very serious problem" of children potentially passing on the virus to older grandparents.
'Hope to turn it around by Christmas'
Matt Hancock said the rules would be in place for the "foreseeable future".
"I really hope we can turn this round before Christmas. I think that, in a pandemic, Christmas is a long way off," he told BBC Radio 4.
"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 in places like Bolton... I very much hope therefore this can work to do that by Christmas."
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Mr Hancock was also asked about testing, after people with symptoms struggled to access testing in recent days.
"Unfortunately we have seen this quite sharp rise in the last couple of weeks of people without symptoms who don't have a good reason coming forward and getting a test," he said, saying 25% of people who were getting a coronavirus test were not eligible.
"I've even heard stories of people saying 'I'm going on holiday next week therefore I'm going to go get a test'. No. That's not what the testing system is there for."
At-a-glance: What are the new rules?
- Social gatherings of more than six people in England will not be allowed in law from Monday 14 September
- The new rule applies to people in private homes, indoors and outdoors, and places such as pubs, restaurants, cafes and public outdoor spaces
- It applies to all ages
- The rule does not apply to schools and workplaces, or weddings, funerals and organised team sports
- A full list of exemptions is due to be published before the law changes
- People who ignore police could be fined £100 - doubling with each offence to a maximum of £3,200
Professor Sir Jeremy Farrar, who sits on the government's scientific advisory committee, said the next four to six weeks "will be absolutely pivotal to the sort of autumn and winter we have".
"So I am very supportive of the restrictions coming in and sincerely hope they are going to be enough."
There are some exceptions to the new rules. Households and support bubbles bigger than six can socialise together - but not with anyone else at the same time - and gatherings can be more than six if it is for work or education purposes.
The Archbishop of Canterbury, Justin Welby, said the new rules did not affect places of worship as that is "not a social gathering".
And Parkrun - which earlier this week announced it was to resume in England - is "unaffected" as it is an organised sport, its organisers said.
No 10 said any group of seven or more people gathering anywhere "risks being dispersed by police or fined for non-compliance".
BBC political correspondent Nick Eardley pointed out that pubs and restaurants would be allowed to have more than six customers inside, but that the groups of six would have to be socially distant from each other.
The rationale behind allowing this, but not allowing larger groups of people inside other people's homes, is that businesses can only be open if they follow safety and hygiene measures set out by the government, he added.
Downing Street said Mr Johnson held a virtual roundtable with police forces last week, and heard officers wanted clearer rules and enforcement on social contact.
The devolved administrations in Scotland, Wales and Northern Ireland are able to set their own coronavirus restrictions and, while largely implementing similar rules, have moved at their own pace during the pandemic.
The number of people allowed to meet inside or outside varies in the UK's four nations. If you are meeting indoors: up to eight people from three different households can meet in Scotland; up to six people from two households in Northern Ireland; up to four households can form an "extended household" in Wales.
Overall, there have been 8,396 new cases reported since Sunday - with 2,460 reported on Tuesday alone.
There were also 32 deaths reported, but these will not have been related to the most recent rise in cases.
Large gatherings 'greatest risk'
The banning of gatherings of over six people is partly about solving a specific problem - giving police powers to crackdown on large gatherings.
It also provides ministers and officials the opportunity to warn the public against complacency with signs infections may be on the rise.
But what is more important is where the UK goes from here.
Lockdown bought us time, but has essentially only deferred the problem - how to protect people, particularly the most vulnerable, from the virus while keeping society functioning.
Better treatments, more testing and a network of contact tracers has put the UK in a better position than it was in the spring - even though there are problems with the last two.
Further restrictions of more or less any sort come at a cost - whether to health, education, the economy or all three.
It means the UK, like all nations, is faced with the impossibly difficult task of balancing harms.
With winter around the corner - the time when respiratory viruses thrive - the decisions are just going to become even more difficult.
The UK government's chief medical adviser, Prof Chris Whitty, who No 10 said backed urgent action in England, has said Covid-19 rates are now rising, especially among people between the ages of 17 and 29.
He warned that if people stopped social distancing then "Covid comes back".
England's deputy chief medical officer Prof Jonathan Van Tam has also voiced concern saying: "People have relaxed too much. Now is the time for us to re-engage, and to realise that this is a continuing threat to us."
In other developments:
- Businesses in England that are required to close because of coronavirus restrictions will be able to claim £1,500 per property every three weeks, the Treasury announced. But the British Chambers of Commerce said for most "this will not be enough to offset the resulting cash crunch"
- A new public information campaign will be launched across the UK reminding people of the "basics" - washing their hands, covering their face and giving others space
- Doncaster Racecourse has been told by public health officials to stop spectators attending its meeting from Thursday. More than 2,500 spectators are expected there on Wednesday
- The quarantine rule on travellers coming into England from seven Greek islands came into force at 04:00 on Wednesday
- Lockdown measures are being reintroduced in Bolton, Greater Manchester, after a local rise in cases. It includes a ban on people socialising with people outside their own household in any setting, even outdoors
- The first clinical trials for a coronavirus vaccine at Oxford University have been put on hold after a participant had an adverse reaction.
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