Schools could be hit by coordinated strikes by heads and teaching staff in the autumn as unions vow to join forces.
Leaders of the four major teaching unions met today to discuss joining forces in the dispute over the Government's "derisory" pay offer.
The National Education Union (NEU) told Education Secretary Gillian Keegan that it will re-ballot its members next month to extend its strike mandate into the autumn.
Tens of thousands of NEU members have already taken part in four national walkouts, with another strike planned for Tuesday.
School leaders in ASCL and the NAHT unions are also being balloted for walkouts, along with the NASUWT teachers union.
NAHT leader Paul Whiteman accused Ms Keegan of showing "contempt" for kids by refusing to restart pay talks after the Government's offer was emphatically rejected by hard-pressed school staff.
The Tory schools chief has washed her hands of the row and said the dispute should go to the pay review body.
In a speech to the NAHT annual conference, Mr Whiteman said: "The Government must now come back to the table. Simply taking their bat and ball home because our members have said no is not the action of a responsible government.
"It is a demonstration of contempt for the profession and a lack of concern for children."
He called for urgent investment in education, adding: "No more empty promises, no more smoke and mirrors with numbers, a genuine financial investment.
"We’ve been on this cliff edge for too long and enough is enough."
Mr Whiteman was joined by the leaders of the NEU, ASCL and NASUWT unions - which have all rejected the Government's pay offer.
He said: "Any illusions they may have had that we would be put off by their derisory first offer and refusal to continue to negotiate should be shattered."
Kevin Courtney, joint general secretary of NEU, said: “In washing her hands of any responsibility for resolving the teacher pay and funding dispute the Education Secretary has united the teaching profession in its determination to not accept the poor offer currently on the table.
"Investment in this generation of children’s education, with professionals shown the value and respect they deserve, should be a priority.
"Parents and the education profession will be in no doubt that if further industrial action needs to be taken the blame for this will lie squarely at the Government's door.”
The Education Secretary held meetings with all four unions before the Government made its pay offer - despite the NEU being the only one with an active strike mandate.
A Department for Education spokesperson said: “For unions to coordinate strike action with the aim of causing maximum disruption to schools is unreasonable and disproportionate, especially given the impact the pandemic has already had on their learning.
"Children’s education has always been our absolute priority and they should be in classrooms where they belong.
“We have made a fair and reasonable teacher pay offer to the unions, which recognises teachers' hard work and commitment as well as delivering at additional £2 billion in funding for schools, which they asked for.”
