England's biggest teaching union is advising members of their "legal right" not to return to their classrooms next week amid serious health and safety fears as Covid cases rise.
The National Education Union (NEU) is telling teachers that their right to work in safe conditions means they cannot be available to take full classes next week.
The union is said to expect that most of its members will follow its advice, forcing most schools to switch to online learning for the majority of pupils.
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The NEU will provide its members with a template letter to send to their headteachers, explaining that they are refusing to go into work because their workplace is unsafe, a right enshrined in law by section 44 of the Health and Safety at Work Act.
Staff should still be prepared to work remotely, the union says, and should volunteer to look after vulnerable pupils and the children of key workers on school premises.
The move came as the NAHT school leaders' union joins the NEU in calling for all schools to move online at the start of term.
And both heads unions have begun a legal process to force the DfE to release all the scientific advice it is drawing on over school Covid safety and to demonstrate that it has given full and proper consideration to the health and safety of pupils and staff.
NEU joint general secretary Kevin Courtney said: "We are doing our job as a union by informing our members that they have a legal right to refuse to work in unsafe conditions which are a danger to their health and to the health of their school communities and more generally...
"In order for viral levels in children and in the community to decrease to below R1 primary schools should not open in the first weeks of January.
"We will be informing our members that they have the right to work in safe conditions which do not endanger their health.
"This means that they can be available to work from home and to work with key worker and vulnerable children but not available to take full classes from Monday, the 4th of January.
"We realise that this late notice is a huge inconvenience for parents and for head teachers.
"The fault, however, is of the government’s own making and is a result of their inability to understand data, their indecisiveness and their reckless approach to their central duty – to safeguard public health.
"Education is really, really important but you’re not going to get that education if this virus gets out of control in the community as schools will have to close then for a longer period of time."
The advice follows a major government u-turn yesterday when all primary schools in London were told to teach online for the start of term, after several boroughs were omitted from a list of areas due to teach remotely.