Boris Johnson has placed the UK on lockdown to tackle the coronavirus, threatening police fines for anyone who ignores new measures including a ban on public gatherings of more than two people.
The Prime Minister detailed a short list of reasons why individuals can leave their homes as he ordered the immediate closure of all shops selling non-essentials items on Monday evening.
He ordered people to only leave the house to shop for basic necessities "as infrequently as possible" and to perform one form of exercise a day.
Or they could seek medical help, provide care to a vulnerable person or travel to work if "absolutely necessary", he said in a televised address from within Downing Street.
"That's all – these are the only reasons you should leave your home," he said.
"You should not be meeting friends. 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.
"If you don't follow the rules the police will have the powers to enforce them, including through fines and dispersing gatherings."
To ensure people follow the rules, Mr Johnson ordered the immediate closure of non-essential stores including those selling electronics and clothing.
All public gatherings of more than two people – other than those you live with – will be barred, the PM said.
Other premises being shuttered are libraries, playgrounds, outdoor gyms and places of worship.
And, while parks will remain open for exercise, all social events including weddings and baptisms will be stopped. Funerals, however, can continue.
Mr Johnson said the measures will be "under constant review" and will be considered for relaxation in three weeks' time if the evidence allows.
He said that "no prime minister wants to enact measures like this" as he reminded the public of the support programme to aid ailing businesses and struggling individuals.
But he said the drastic new measures allowing people to only leave home for the "very limited purposes" were necessary to slow the spread of the disease.
"To put it simply, if too many people become seriously unwell at one time, the NHS will be unable to handle it – meaning more people are likely to die, not just from coronavirus but from other illnesses as well," he added.
Mr Johnson had been facing widespread calls to impose tough restrictions on the nation amid concerns people are ignoring social distancing advice.
His call came after the UK death toll hit 335 and British citizens travelling abroad were told to return home "while you still can".
Forty-six more people died in England alongside four in Scotland and four in Wales, taking the number who have died in British hospitals after testing positive to 335. Those who have died in England range in age from 18 to 105.
In an earlier escalation of advice, Foreign Secretary Dominic Raab told citizens travelling overseas to return to the UK using commercial routes that are still running.
"If you are on holiday abroad the time to come home is now while you still can," he said.
Foreign Office staff were working to help citizens get back where routes have been halted due to the growing crisis.
Meanwhile, MPs were debating emergency legislation that could see airports shut and police having the powers to force people with virus symptoms to isolate.
And, elsewhere, WHO general director Dr Tedros Ghebreyesus said the pandemic is "accelerating" as he urged nations to impose more stringent measures than social distancing advice.
"To win, we need to attack the coronavirus with aggressive and targeted tactics – testing every suspected Covid-19 case, isolating and caring for every confirmed case, and tracing and quarantining every close contact," he said.
Over the weekend, crowds of people were witnessed visiting open spaces across many parts of the UK, at times flouting official social distancing advice.
Health Secretary Matt Hancock had earlier called those who disregarded the advice "very selfish", as he discussed measures to support NHS workers.
He told BBC Radio 4 that he hoped testing of medical staff for Covid-19 would take place "as soon as possible".
And he said the Government has confirmed that all major hospitals have received deliveries of personal protective equipment (PPE) needed by frontline staff and that a hotline has been set up for NHS workers to ring if there are shortages in their area.
Mr Hancock also said that 12,000 ventilators are now available after last week's appeal by the PM, up from 5,000 that the NHS had access to previously.
The severe measures came as Wendy Jacobs, a head teacher at Roose Primary School in Barrow-in-Furness, Cumbria, died after testing positive for coronavirus.
In Walsall, a 36-year-old nurse and mother-of-three was on a ventilator in intensive care after contracting coronavirus.
Areema Nasreen was in a critical condition at Walsall Manor Hospital in the West Midlands, where she works.
There were 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.
The Italian government was one of a number of European countries to announce new or extended restrictions – with Germany banning public gatherings of more than two people not from the same household.