All shops selling non-essential goods will be closed, and gatherings of more than two people from different households will be banned.
“No Prime Minister wants to enact measures like this,” Mr Johnson said in an address to the nation last night.
It came as the UK death toll hit 335 and the first cases emerged of medical staff being struck down with the illness.
Places of worship, libraries, playgrounds and outdoor gyms will be closed. Parks will remain open, but groups will be dispersed, and all public ceremonies except for funerals will be banned.
The Prime Minister said the only permitted reasons for leaving the house will be shopping for basic necessities “as infrequently as possible”, doing one form of exercise a day alone or with members of your household, getting medicine or going to the doctor, providing care or help to a vulnerable person, and travelling to work “only where this is absolutely necessary and cannot be done from home”.
“You should not be meeting friends,” Mr Johnson said. “If your friends ask you to meet, you should say No. You should not be meeting family members who do not live in your home.
“You should not be going shopping except for essentials like food and medicine — and you should do this as little as you can. And use food delivery services where you can.
“If you don’t follow the rules the police will have the powers to enforce them, including through fines and dispersing gatherings.”
Three week period
The Prime Minister said the measures would be reviewed after three weeks, and relaxed “if the evidence shows we are able to”.
There was anger over the weekend as photos emerged showing crowds of people visiting open spaces across many parts of the UK, in spite of strong advice to stay at home.
And in London, where the coronavirus outbreak is peaking fastest and half of all UK deaths are concentrated, pictures of packed tube carriages yesterday morning added to calls for tougher measures to be introduced, with Labour demanding “enforced social distancing”.
There are growing fears that Britain is on a similar trajectory to Italy - scene of the world’s worst outbreak - where the death toll passed 5,000 over the weekend.
However, after two weeks of lockdown, Italy has recorded a second day of decline in the number of deaths, and the fourth straight day of declining new cases of coronavirus, raising hopes that tough measures are finally having an effect.
Speaking in the Commons, the former Health Secretary Jeremy Hunt called for an immediate nationwide lockdown.
“It may be too late to avoid Italy, but to have any chance at all of doing so, we must move to lockdown rules now that ban non-essential travel,” Mr Hunt said.
“It is time not just to ask people to do social distancing, but to enforce those social distancing rules - not next week, not this week, but right away.”
‘People have been selfish’
Health Secretary Matt Hancock had earlier warned that the government was willing to take “more action” if needed to stop coronavirus from spreading, and said those ignoring official health advice were being “very selfish”.
“If people go within two metres of others who they don’t live with then they’re helping to spread the virus - and the consequences of that costs lives and it means that, for everyone, this will go on for longer,” Mr Hancock told the BBC.
A 36-year-old nurse and mother-of-three is on a ventilator in intensive care in the West Midlands after contracting coronavirus.
Areema Nasreen is in a critical condition at Walsall Manor Hospital, where she works.
Seven doctors are also reported to have contracted the disease at the same London hospital.
Mr Hancock said yesterday that he hoped widespread testing of medical staff would be rolled out “as soon as possible”.
And he said the Government has confirmed that all major hospitals have received deliveries of personal protective equipment (PPE) as of the weekend, with a hotline set up for NHS workers to ring if there are shortages in their area. Mr Hancock also said 12,000 ventilators are now available after an appeal.
Labour’s shadow health secretary Jonathan Ashworth said: “After another weekend of apparent public confusion and widespread non-compliance with ‘social distancing’, of grave scientific warnings and brave medical professionals talking of being sent to work like ‘lambs to the slaughter’ with inadequate protective equipment, something has to change.”
Meanwhile, the government said it would amend emergency coronavirus legislation so sweeping powers will have to be renewed by a vote in the Commons every six months.