Hundreds of thousands of teachers will be balloted for strike action over pay in a move that could bring major disruption to schools this winter.
The NAHT, which represents heads, is expected to announce this week that it will ballot its members in England for strike action over pay for the first time in its 125-year history.
Schools could be forced to close if heads walk out, as many heads and deputy heads hold the necessary safeguarding roles to keep the doors open.
It comes after the NEU, the country's largest education union, and the NASUWT, which represents teachers, both confirmed on Friday that they would launch formal strike ballots.
Teachers are up in arms over the Government's below-inflation pay offer, under which experienced staff would get 5% pay hikes and newer staff see their wages rise by 8.9%.
But with inflation hovering around 10%, unions say the offer would amount to a real terms cut to wages.
The NAHT consulted its 34,500 members over the pay award, with the majority understood to be unhappy with what's on the table.
General Secretary Paul Whiteman told the Sunday Times: "The level of anger and desperation I am hearing from our members is unprecedented.
"Rising costs and lack of funding are forcing school leaders to make cuts they know will negatively impact the education and wellbeing of children and young people."
The NEU confirmed on Friday that it would hold a formal strike ballot in England and Wales of around 300,000 school staff, after 86% of its teachers and 78% of support staff backed action in preliminary consultations.
Joint General Secretaries Dr Mary Bousted and Kevin Courtney said: "Successive Conservative governments have failed our members, our public services, and our children.
"The latest pay rise, which the present Government refuses to fully fund, will simply not do. Our members expect a fully funded, above-inflation pay rise and have today demonstrated their willingness to consider strike action in pursuit of one."
The NASUWT has also said it will ballot its members in Scotland, England and Wales.
General Secretary Patrick Roach said: “The Government has failed to recognise the damaging impact of years of real terms pay cuts on the morale of teachers and which is fuelling the teacher recruitment and retention crisis.
“The 5% pay award for teachers and headteachers is unacceptable at a time when inflation is running at more than 10% and it will result in even more financial misery for hard working teachers."
A Department for Education spokesperson said: “It is incredibly disappointing that some unions are threatening industrial action in schools.
"Strike action will damage children’s education and disrupt parents’ lives. Given the impact of the pandemic on children, it’s more important than ever that strike action is avoided.
“We have confirmed the highest pay awards for teachers in a generation – 8.9% for new teachers and five percent for experienced teachers and leaders - recognising their dedication and hard work."
