A pay review which aims to ensure that no public sector manager earns more than 20 times that of their lowest-paid worker will not include headteachers, it has emerged.
However Chris Keates, general secretary of teaching union the NASUWT, said the growth of academies - which are free from restrictions on heads' pay - made it even more important that schools were included in the review.
But headteachers' leader John Dunford, general secretary of the Association of School and College Leaders, has hit back, claiming that most teachers' and heads' pay is set transparently by the school teachers' review body.
Excessive pay levels, even for academy heads, are restricted by school budgets, he added.
A recent analysis by The TES estimated that at least 100 heads are earning more than #163;150,000. The lowest-paid support worker would have to earn less than #163;7,500, full time, for the salary limit to kick in.
However, Ms Keates said of the review, due to be carried out by economist and commentator Will Hutton: "The exclusion of schools from this review raises serious questions. If transparency and fairness of salary levels is an important principle governing public sector pay, then there can be no credible rationale for excluding from the review the comparative pay levels between headteachers and other staff."
She also suggested that the Government was trying to seduce heads into opting for academy status.
She added: "Cynics might say that the exclusion of schools is part of a deliberate and calculated Government strategy to flatter and woo headteachers to the policy of free schools and academies.
"These independent schools will be given control over millions of pounds of taxpayers' money. Taxpayers have a right to know how it will be spent. It is simply unjustifiable and unacceptable that schools are excluded."
Mr Dunford responded, saying: "Teachers have a pay review body which has constantly kept under review the relative pay of school leaders and classroom staff and it shows a lack of trust in that body to suggest that headteachers are somehow given more favourable treatment than others.
"They earn more because their responsibilities and activities are so great."