Prime Minister Boris Johnson said that school closures across the country were necessary from Friday for most pupils, to slow the spread of coronavirus.
"We must apply downward pressure, further downward pressure on the upward curve by closing the schools," Johnson said at a news conference on Wednesday.
"After schools shut their gates from Friday afternoon, they will remain closed for most pupils, for the vast majority of pupils, until further notice."
Johnson said the government was keeping everything under review when asked whether he would bring in measures to lock down London because people were flouting warnings to avoid social gatherings.
The government has advised people not to go to pubs, restaurants, cinemas or restaurants to help stop the spread of coronavirus.
Asked whether tougher measures were needed to shut down London where bars, public transport and businesses remained busy, Johnson said: "We've always said we're going to do the right measures at the right time.
"A lot of people are making a real heroic effort to comply with the advice we've given, but as I've said ... we keep everything under continuous review and we will not hesitate to bring forward further and faster measures where that is necessary."
The majority of schools will close from Friday, although some will be asked to remain open to support the children of essential workers like health care employees, education minister Gavin Williamson told parliament.
"I know the situation has become increasingly challenging. I said before that if the science and the advice changed such that keeping schools open would no longer be in the best interests of children and teachers that we would act," said Williamson.
"We are now at that stage. The spike of the virus is increasing at a faster pace than anticipated."
The shutting of so many schools will have massive economic and social repercussions for the world's fifth-biggest economy. It will alter the lives of almost 9 million British children and force parents to stay home from work to look after them.
The decision came after headteachers were increasingly forced to turn away pupils by either closing or partially closing schools because of staff shortages.
Earlier, Scotland announced it would close its schools from Friday and the government in Edinburgh said there was no guarantee they will reopen before the summer.
"We will of course only keep them closed for as long as we absolutely have to," said Scotland’s first minister Nicola Sturgeon. "But at this stage, I cannot promise that they will reopen before the summer holidays."
Schools on Wednesday began writing to parents asking if they worked in sectors that will be required to carry on through the peak of the crisis such as medical professionals or care workers.
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