Boris Johnson has stunned millions of Britons with a momentous and tough lockdown announcement to tackle coronavirus, warning people to stay at home or face a fine.
In a dramatic and historic TV address to the nation, the prime minister said all shops apart from food stores and chemists must close immediately, meeting friends will be banned - and just one form of exercise a day will be allowed.
The COVID-19 lockdown, which will be in force for at least three weeks, follows growing pressure in recent days from the government's top medical advisers and senior politicians.
Announcing powers not seen since wartime, Mr Johnson declared: "From this evening I must give the British people a very simple instruction - you must stay at home.
"Because the critical thing we must do is stop the disease spreading between households."
People will only be allowed to leave their home for "very limited" purposes:
He added: "That's all - these are the only reasons you should leave your home.
"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.
"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."
Mr Johnson added that these measures will come into force immediately:
"Parks will remain open for exercise but gatherings will be dispersed," he continued.
Explaining why he has taken such strong action, Mr Johnson said: "Without a huge national effort to halt the growth of this virus, there will come a moment when no health service in the world could possibly cope; because there won't be enough ventilators, enough intensive care beds, enough doctors and nurses.
"And as we have seen elsewhere, in other countries that also have fantastic healthcare systems, that is the moment of real danger.
"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.
"So it's vital to slow the spread of the disease.
"Because that is the way we reduce the number of people needing hospital treatment at any one time, so we can protect the NHS's ability to cope - and save more lives.
"And that's why we have been asking people to stay at home during this pandemic.
"And though huge numbers are complying - and I thank you all - the time has now come for us all to do more."
Mr Johnson, who has previously attacked the so-called "nanny state", acknowledged: "No prime minister wants to enact measures like this.
"I know the damage that this disruption is doing and will do to people's lives, to their businesses and to their jobs.
"I can assure you that we will keep these restrictions under constant review. We will look again in three weeks, and relax them if the evidence shows we are able to.
"But at present there are just no easy options. The way ahead is hard, and it is still true that many lives will sadly be lost.
"And yet it is also true that there is a clear way through.
"Day by day we are strengthening our amazing NHS with 7,500 former clinicians now coming back to the service.
"With the time you buy - by simply staying at home - we are increasing our stocks of equipment, we are accelerating our search for treatments, we are pioneering work on a vaccine, and we are buying millions of testing kits that will enable us to turn the tide on this invisible killer."
And he concluded: "I want to thank everyone who is working flat out to beat the virus. Everyone from the supermarket staff to the transport workers to the carers to the nurses and doctors on the frontline.
"But in this fight we can be in no doubt that each and every one of us is directly enlisted. Each and every one of us is now obliged to join together to halt the spread of this disease and to protect our NHS and to save many many thousands of lives."
The PM's dramatic announcement came as the number of coronavirus cases in the UK rose to 6,650 in the latest 24 hours, with the number of deaths from the disease up by 55 to 336.
It also follows a sunshine-bathed weekend in which thousands of people ignored official warnings to stay at home and Mr Johnson vowed to consider tougher action within 24 hours.
He carried out his threat after chairing a top level COBRA meeting of senior ministers, medical advisers and scientists - conducted via video conference to avoid risk - as he addressed TV viewers directly from inside 10 Downing Street.
Meanwhile, MPs rushed emergency legislation through the Commons in a single day, giving the government sweeping powers which could be on the statute book for two years.
The emergency legislation includes powers to shut airports and allow police to force people with virus symptoms to isolate, with the threat of £1,000 fines for those who refuse to take a coronavirus test.
Having completed its passage through the Commons, the Bill will be debated in the House of Lords on Tuesday and Wednesday and the Government hopes there will be no Lords amendments so it can receive royal assent and become law by Thursday.
Earlier Foreign Secretary Dominic Raab advised all British travellers to return to the UK now, before airline routes, land borders and other restrictions make it impossible.
He said: "Around the world, more airlines are suspending flights and more airports are closing, some without any notice.
"Where commercial routes don't exist, our staff are working round the clock to give advice and support to UK nationals. If you are on holiday abroad the time to come home is now while you still can."