It comes after the Association of School and College Leaders (ASCL) called on the government to immediately halt all Ofsted inspections on Saturday.

The watchdog has said it will defer visits to any school or college affected by the virus.

But the letter, from the NEU’s general secretaries, said: «We are very concerned, given the serious challenges that Covid-19 poses for schools, that Ofsted is still planning to conduct inspections.

“We fail to understand that, in light of the government’s decision to move to the ‘delay’ phase, Ofsted has revised its deferral policy to defer inspections on providers that are ‘affected’.

“We have to ask the question – can you name any school in England which has not been affected by Covid-19?”

It adds: “Even if there are no reported cases in schools, all leaders and staff are highly alert and responding to a range of pressing concerns and issues about the management of Covid-19, all of which involve changes to school routines and an intensification of already excessive workloads.”

Addressing hundreds of headteachers at its annual conference in Birmingham, Geoff Barton, general secretary of ASCL, said it was not a time for “business as usual” but a time of “national emergency”.

Mr Barton also called on Ofsted to show it understands the “extraordinary pressures” on schools. One school leader, he said, currently has 15 members of staff off because of coronavirus.

“It simply cannot be right that schools and colleges are judged in a high-stakes manner in these extraordinary circumstances,” he said.

Education secretary Gavin Williamson was due to meet with headteachers’ unions on Monday to discuss the variety of concerns that school leaders have over coronavirus.

Addressing the ASCL conference, Sean Harford, Ofsted’s national director of education, said: “We will be proactive in asking schools and colleges if they want to request deferral because of coronavirus and clearly we will look very favourably on all such requests.”

The chief inspector has asked to be “personally involved” in any decision not to defer, Mr Harford said.

Speaking to the media, Mr Harford added Ofsted was «not able to say that we’re ceasing inspections, that’s something for the government to decide» and would need «legislative change» to do so.