Britain on Wednesday ordered schools, nurseries and colleges to close for millions of children until further notice after criticism the government was being too slow to react to the spread of coronavirus.

Most schools will close from Friday, although some will be asked to stay open to support the children of essential workers like healthcare 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," he said.

"We are now at that stage. The spike of the virus is increasing at a faster pace than anticipated."

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The decision came after headteachers were increasingly having to turn away pupils anyway because of staff shortages.

Geoff Barton, general secretary of the Association of School and College Leaders union, earlier said headteachers at large schools were saying it would be a "struggle" to keep their institutions open beyond Friday. Between 10-20 percent of teachers are currently self-isolating, but the number is rising, he said.

"It is becoming harder to sustain the staffing in those schools, some schools are having to send home year groups, and some schools are starting to close," he added.

The shutting of so many schools will have huge economic and social repercussions for the world's fifth-biggest economy. It will alter the lives of almost nine million British children and force parents to stay home from work to look after them.

The move to shut schools is a reversal of the government's recent cautious stance towards tackling the virus.

Britain had previously resisted pressure to follow the lead of Italy, France, and Spain, saying school closures would not halt the outbreak and would deprive the country of key public sector workers.

Kevin Courtney, joint General Secretary of the National Education Union, welcomed the government's announcement.

"It is better for this to take place in an ordered way than the chaotic pattern of closures that was developing," he said.

'A pparent contradiction '

Neil Ferguson, a professor of mathematical biology at Imperial College London, whose modelling of the spread of the coronavirus swayed the government's response, has said school closures would reduce the transmission of the disease.

In a letter to Prime Minister Boris Johnson, the National Education Union said there was an "apparent contradiction" between banning mass gatherings and keeping schools open.

School leaders have already begun making contingency plans to ensure vulnerable children and children of public sector workers will still be looked after.

Ruth Luzmore, a head teacher in London, closed her primary school to everyone but pupils whose parents who work for the police, fire service, or are teachers at other schools that have not closed.

The school closure comes after the number of deaths of people who tested positive for coronavirus in England rose to 99, a 48 percent jump from the day before.