Head teachers could take industrial action over the schools funding crisis.
The National Association of Head Teachers annual conference has backed a motion committing the union to explore options to challenge the funding crisis up to and including industrial action.
Heads also voted to refuse to make staff redundant or reduce their terms and conditions to balance budgets.
Proposing the motion, London head Dave Woods said cuts risked the education of an entire generation of children.
"It is not good enough," Mr Woods told the conference in Telford.
"This crisis has reached a point where schools are no longer able to provide standards of education afforded to previous generations of this country...
"Despite our loud and continuous warnings and those from other school leaders, parents, governors, the National Audit Office, local and district councils, I could go on and on - the government not only refuses to listen but deny there is a problem."
Staffordshire head teacher Emily Proffitt said she celebrated the fact that heads were no longer sitting in their offices "with their heads buried in their hands, wondering how they are going to cut their cloth".
"There is only so much cloth you can cut before you are fully exposed and the mending is impossible," she added.
Ms Proffitt also said the union should take all steps necessary to achieve the aim of a 10 year funding plan for schools.
But another head, Miranda Gibbs, said the conference resolution would leave her with divided loyalties as her school's governing body was very clear that she should operate a balanced budget, and the school had already "very reluctantly" begun redundancy procedures for some staff.
'Loud and clear'
The motion also called for school leaders to set and publicise projected budgets to reveal the actual amounts they need to run their schools compared with the money available in reality.
Earlier this year, about 7,000 head teachers in England wrote to parents highlighting a "funding crisis" in their schools.
And last month, research by the NASUWT teachers' union suggested that one in five teachers was using their own money to buy classroom resources.
Education Secretary Damian Hinds told the conference he had heard concerns about funding "loud and clear".
Addressing the heads on Friday, he said: "I know finances are challenging for many schools - that you have had to make and do have to make hard decisions about resources."
He said he would be setting out the case for better funding for education in the government's upcoming Comprehensive Spending Review.
"The case for investment in education is a special one, because so much else depends on what you do - so much for economy and our society."
Closing the conference, NAHT general secretary Paul Whiteman said the task of head teachers was to "create the best conditions" in their schools to allow children "the very best education possible".
"We will work with government, we will advise, and we will campaign," said Mr Whiteman.