Social gatherings of more than six people will be illegal in England from Monday in the biggest coronavirus crackdown since lockdown rules were eased.
First offenders will be fined £100, which will double on each further repeat offence up to £3,200, the prime minister announced, in a bid to curb the surge in Covid-19 cases in the UK.
There will be some exemptions and will not apply to schools, workplaces or Covid-secure weddings, funerals and organised team sports.
The new rules - which come into force on 14 September - mark a change to England's current guidance.
"It is absolutely critical that people now abide by these rules and remember the basics - washing your hands, covering your face, keeping space from others, and getting a test if you have symptoms," the prime minister will say.
"We need to act now to stop the virus spreading," Mr Johnson will declare at a Downing Street news conference, coinciding with a new government advertising campaign entitled "Hands. Face. Space."
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.
The full list of exemptions will be revealed by the government before the law changes on Monday.
Pubs, restaurants and other hospitality businesses will also be forced to collect customers' details to aid with contact tracing, Health Secretary Matt Hancock has said, instead of it just being voluntary.
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".
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.