Teaching unions have warned the government that primaries are not "Covid secure" and need to be made safer.
But in an explanation of its latest U-turn on school Covid closures - that will see all London primaries move online from Monday - the Department for Education says the issue is not about safety in schools, but about the virus "in the community".
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The full statement reads: "Restrictions on primary schools will be introduced across London.
"The government has today, Friday 1 January, announced that the education contingency framework will be implemented across all London boroughs, following a further review of the transmission rates.
"In light of Covid case rates rising rapidly across the capital and ongoing engagement with London leaders and the evidence submitted, the government has reviewed the London boroughs where the contingency framework will apply, with all further boroughs added.
"The contingency framework is being implemented across London as a last resort to help suppress the spread of the virus in the community, protect public health and save lives.
"The framework is not being implemented due to safety concerns in education. Schools have well established protective measures in place to maximise safety for pupils and staff and help reduce the risk of transmission.
"Decisions on which areas are subject to the contingency framework are taken working closely with Public Health England, Department of Health and Social Care and the NHS.
"Due to the fast moving situation, and where local conditions are changing rapidly, the review of London boroughs was brought forward for a decision today as part of the contingency framework process.
"From Monday 4 January, London primary schools will be required to provide remote learning to all children but vulnerable and critical worker children who will continue to attend school.
"Early years will remain open in London, as will alternative provision and special schools."
Education secretary, Gavin Williamson said: “Children’s education and wellbeing remains a national priority.
"Moving further parts of London to remote education really is a last resort and a temporary solution.
“As infection rates rise across the country, and particularly in London, we must make this move to protect our country and the NHS.
"We will continue keep the list of local authorities under review, and reopen classrooms as soon as we possibly can.”
Health secretary Matt Hancock said: “Over the past week we have seen infections and hospitalisations rise sharply across London and hospitals are coming under increased pressure.
“While our priority is to keep as many children as possible in school, we have to strike a balance between education and infection rates and pressures on the NHS.
“The situation in London continues to worsen and so today we are taking action to protect the public and reduce the spread of this disease in the community.
“Everyone across London must take this situation incredibly seriously and act responsibly to minimise the spread of this deadly disease.”