The prime minister is announcing strict new curbs on life in the UK to tackle the spread of coronavirus.
From this evening people must stay at home except for shopping for basic necessities, daily exercise, any medical need and travelling to and from essential work.
Shops selling non-essential goods will also be shut and gatherings in public of more than two people who do not live together prohibited.
The UK death toll has reached 335.
If people do not follow the rules police will have the powers to enforce them, including through fines and dispersing gatherings, Boris Johnson said in a televised statement from Downing Street.
Other premises including libraries, playgrounds and outdoor gyms, and places of worship must also close immediately.
Parks will remain open for exercise but gatherings will be dispersed.
The government is also stopping all social events, including weddings, baptisms and other ceremonies – but funerals will be allowed.
Mr Johnson said the restrictions would be kept under constant review.
“We will look again in three weeks, and relax them if the evidence shows we are able to,” he added.
The prime minister said the measures were necessary to tackle “the biggest threat this country has faced for decades”.
“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,” he said.
“And as we have seen elsewhere, in other countries that also have fantastic health care systems, that is the moment of real danger.”
The UK has been under growing pressure to follow other countries by ordering the closure of more shops, and enforcing rules on social distancing, amid concerns people have been ignoring government advice.