The prospect of a public sector pay freeze will be the "final straw" for experienced teachers, and could lead to an "exodus from the profession", heads have warned.
If the government freezes salaries at the current level despite the Covid pandemic, it risks "precipitating a crisis" in which many teachers decide "enough is enough", according to the Association of School and College Leaders (ASCL).
Geoff Barton, ASCL general secretary, was responding to reports that chancellor Rishi Sunak is preparing to impose a pay cap on five million public sector workers as he seeks to rebuild the public finances.
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The news follows the publication of a new report from a public sector think tank arguing that the chancellor could save up to £23 billion if he were to impose a three-year public sector pay freeze.
The Centre for Policy Studies (CPS) warned that without action to curb pay, the government will face a sharply rising public sector wage at a time of increased borrowing due to the pandemic.
CPS director Robert Colvile said: "The economic impact of the Covid-19 pandemic has been severe but the pain has not been shared equally.
"Healthcare workers aside, it is difficult to justify generous pay rises in the public sector when private sector wages are actually falling.
"At the same time, there is a need to control public spending and reduce the structural deficit which the pandemic is likely to have opened up."
But Mr Barton said "there is only so much that people can take".
"For many experienced teachers and leaders the prospect of a pay freeze will be the final straw and we are extremely concerned that it will lead to an exodus from the profession," he said.
"They have worked under relentless pressure during the coronavirus pandemic and are on the frontline of managing public health measures while also delivering education.
"In addition, their pay has not recovered from a decade of austerity, their schools are underfunded, and they are subjected to an excessively harsh accountability regime.
"There is only so much that people can take.
"If the government goes down the road of imposing a pay freeze on top of everything else, it is in danger of precipitating a crisis in which many teachers and leaders decide enough is enough."
He added: "It is also infuriating to once again see major announcements affecting the lives of loyal public sector workers being trailed first in the media rather than hearing directly from the government."
Paul Whiteman, general secretary of school leaders' union NAHT, said the news was "a huge kick in the teeth for millions of school staff".
"Undoubtedly the costs of Covid have been significant, but the government is looking for the wrong solution to the problem of balancing the national budget," he said.
"The teaching profession has already endured a decade of pay freezes, in contrast to pay growth in the private sector. This made teaching an uncompetitive career option for graduates who have looked to other sectors to build their careers. The government's plans will make a dire situation even worse.
"The civic response to Covid has been characterised by the willingness of public sector workers, school staff included, to go the extra mile. Public sector workers have been critical to the national effort. They have also endured personal losses and tragedies.
"It is entirely wrong for the government to expect dedicated public servants, some of them often on low rates of pay already, to be the ones to shoulder the responsibility of paying for the costs of Covid incurred by the Treasury.
"Another slap in the face on pay, after years of pay freezes and an unbelievably challenging 2020 is an insult that many school staff will be absolutely stunned by."
The Treasury would not comment on the reports ahead of the chancellor's statement on the spending review on Wednesday.