The Secondary Heads Association and the National Association of Schoolmasters Union of Women Teachers both think the transfer of administrative tasks to support staff could eventually, in some schools, lead to an overall reduction in teaching posts.
They stress the changes will take place over the long term and will not threaten the jobs of any existing teachers.
John Dunford, SHA general secretary, said that transferring administrative tasks to support staff would leave teachers with more teaching time and so fewer of them would be needed. The overall reduction in the teacher workforce would be slight and would happen over time through natural wastage, he said.
Chris Keates, NASUWT, deputy general secretary, said existing teachers'
jobs were not at risk and nobody was setting out to reduce teacher numbers.
She said: "When remodelling starts to impact on schools some of the jobs teachers have done will be done by support staff, and then adjustments will have to take place.
"You might have needed three teachers previously whereas now you may need 2.4 teachers because there are fewer administrative tasks."
But Mary Bousted, Association of Teachers and Lecturers' general secretary, said: "The agreement is about sustaining or increasing numbers of teachers and we won't be party to an agreement implemented in any other way."
John Bangs, National Union of Teachers' head of education, said: "This is a complete vindication of our stand. We always said that the agreement would be used to reduce teacher numbers."
A Department for Education and Skills spokesman said: "Our reforms to the teaching workforce are all about freeing teachers to do what they do best - teach, giving pupils more individual support in the classroom. They are not and never have been about increasing class sizes, reducing teacher numbers or about replacing teachers with unqualified staff."
Only a fortnight ago David Miliband, schools minister, told the SHA conference in Harrogate that claims that the workforce agreement was about cutting teacher numbers were "complete and utter nonsense".
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