Every school in the UK is to close "until further notice" as the nationwide response to tackle the coronavirus crisis steps up.
Boris Johnson confirmed the move – which brings the UK into line with the rest of Europe – as it emerged that the number of people in the country killed by the illness has risen to 104.
Universities, colleges and nurseries are also being urged to follow suit.
The prime minister said that after schools in England "close their gates" on Friday, they will not re-open again until the outbreak is under control.
However, exceptions will be made for the children of key workers, such as those in the NHS and the emergency services, and the most vulnerable youngsters, including those relying on free school meals.
The devolved governments in Scotland and Wales also announced that all of their schools will close at the end of the week, while those in Northern Ireland are shutting with immediate effect.
Johnson said: "Looking at the curve of the disease, and where we are now, we think now that we must apply downward pressure, further downward pressure on that upward curve, by closing the schools.
"So, I can announce today...that after schools shut their gates from Friday afternoon, they will remain closed...for the vast majority of pupils until further notice.
"The objective is to slow the spread of the virus, and we judge this is the right moment to do that.
"But of course, as I've said, we also need to keep the NHS going, and to treat the rising number of cases. So we need health workers who are also parents to continue to go to work and we need other critical workers with children to keep doing their jobs too.
"From police officers who are keeping us safe, to the supermarket delivery drivers, social care workers who look after the elderly."
However, the prime minister urged people not to send their children to be looked after by their grandparents, as the elderly are among the group most at risk from the virus.
As the prime minister addressed a Downing Street press conference, education secretary Gavin Williamson told the Commons that "the spike of the virus is increasing at a faster pace than anticipated and it is crucial that we continue to consider the right measures to arrest this increase and relieve the pressure on the health system".
He added: "The public health benefits of schools remaining open as normal are shifting. It is also clear that schools are increasingly finding it more difficult to continue as normal as illness and self isolation impacts on staffing levels and pupil attendance."
Ahead of a statement by Scottish education secretary John Swinney tomorrow, Scottish first minister Nicola Sturgeon said “this has been one of the hardest decisions we have faced so far as we tackle the coronavirus”.
She added: WSAGE [Scientific Advisory Group for Emergencies] – our expert scientific advisers – are examining new advice that is very likely to tell us to close schools. We also know more and more schools are approaching a point where they have lost too many staff to continue as normal. At this stage I cannot promise schools and nurseries will reopen after the Easter break.
“There will require to be a lot of local flexibility and we are working closely with local authorities to put those arrangements in place.
“Teachers, school staff and those in the nursery sector will have a vital role in the weeks ahead and we will work with you to minimise the impact on all our young peoples’ education, and in particular the most vulnerable groups.”
Welsh education minister Kirsty Williams said she was bringing forward the Easter break for schools in Wales, closing at the latest on 20 March.
“From next week, schools will have a new purpose. They will help support those most in need, including people involved in the immediate response to the coronavirus outbreak. I am working with my colleagues in the Cabinet, with government officials and our partners in local government to develop and finalise these plan,” she said.
“Today’s decision will help ensure an orderly closure, so schools have time to prepare ahead of the early break.
“My main message for everyone is to stay safe and stay well. We will work together and we will face this outbreak together.”
Northern Ireland first minister Arlene Foster acknowledged the move was “unprecedented”.
“Our school principals, parents and pupils have been in a holding pattern based on medical advice for the last week.
“Today we have agreed that all schools will close from Monday 23 March.
The societal and economic impact of this measure will be enormous as parents have to adjust their routine to deal with this unplanned long-term closure. Our medical advice was to delay this step for as long as possible as the closure will likely take us beyond the natural break for summer.
“We are exploring how our schools can continue to be a base for the education of children whose parents are health service staff or other key workers such as the blue light services.”