Heads' leaders have taken the "exceptional step" of writing to the prime minister and the chancellor warning that their plans to overhaul teachers' pensions will derail the Government's school reforms.
In a letter to David Cameron and George Osborne, the Association of School and College Leaders (ASCL) and heads' union the NAHT have spelt out that opposition among heads to the changes is "unprecedented".
Mr Osborne and the Treasury are pushing through reforms to the teachers' pension scheme, which could see employee contributions rise by 50 per cent.
The NAHT has already voted to ballot its members on whether it should take strike action over the issue, joining unions the NUT and the ATL.
In the letter, ASCL general secretary Brian Lightman and NAHT general secretary Russell Hobby state that a prolonged dispute on pensions will "undermine" the ability of the Coalition's policies to succeed.
"You have a number of radical reforms which will require the full engagement of school leaders to implement and show impact - from phonics and the pupil premium to teaching schools and new approaches to behaviour and the curriculum," the letter says. "This engagement is at risk."
The two union leaders say Mr Osborne's pension reforms will lead to a "large cohort" of heads taking early retirement to avoid what they describe as a pay cut, which could ultimately "derail" the academies programme.
The move will "seriously demotivate" school leaders, the letter adds, putting extra pressure on recruitment into teaching and filling headteacher roles, while any dispute will cause further "deterioration" to working conditions in schools.
Under the Government's proposals, contributions would rise from 6.4 per cent of salary to an average of between 9.5 and 9.8 per cent by 201415.
A teacher earning pound;35,000 a year would have to pay an extra pound;100 a month, while pensions would shift from a final-salary scheme to career- average.
Mr Osborne is also looking to gradually raise the retirement age.
Mr Hobby said: "Teachers have already accepted a two-year pay freeze and tighter budgets. We already make significant contributions to our pension scheme and have agreed to bear the risk if it is unaffordable - but there is no evidence that it is unaffordable."
He added: "We know that good teachers, support staff and leaders will leave the profession early if their pension benefits are cut. The bottom line is that the standard of education will suffer because of damage to public sector pensions."
Speaking at the NAHT annual conference this year, education secretary Michael Gove said he acknowledged that teachers had an unspoken contract with the Government on pensions and that he would be "championing" their cause.
A spokesperson at Number 10 said the prime minister would respond in due course.
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Original headline: Our pensions or your reforms: heads take on PM and chancellor