Headteachers' pay is likely to be capped for the first time after a union called for the end of "telephone number salaries" for school leaders.
Teaching unions - including those representing heads - agreed with calls for an upper limit on salaries amid demands for a full review of pay for senior staff, including those working at academies.
However, in evidence to the School Teachers' Review Body (STRB), which advises ministers on pay, a joint submission by six unions rejected education secretary Michael Gove's calls for the cap to be imposed in line with that of the prime minister, who currently earns pound;142,500.
Mr Gove's proposal to use the prime minister as a benchmark was dismissed as "arbitrary", with the unions saying it "patently does not compare like with like".
The NASUWT, which submitted separate evidence, described a cap as a "blunt instrument" and instead called for heads' pay and perks to be published annually. "This in itself imposes a discipline on the setting of salary levels," the union said.
The STRB investigation follows last week's publication of the Government- commissioned fair pay review - led by Will Hutton - which stated that no senior member of staff should earn over 20 times the salary of the lowest- paid worker. Mr Hutton also rejected the trend of using the prime minister's salary as a limit for public sector employees.
Martin Freedman, head of pay and conditions at teaching union ATL, said: "We are not averse in principle to an upper limit, but we'd need a proper review that looked at the changing roles of school leaders over the years."
The joint union submission called for the salaries of academy heads - who are currently paid outside national pay scales - to be brought into line with those at other schools.
Mr Freedman said: "At the moment, academies are advertising telephone number salaries. We want governing bodies to apply objective criteria to pay."
Schools are currently ranked in eight groups according to size, with heads paid accordingly. For challenging schools, governors can offer a salary of up to two groups above the school's "proper" limit.
For schools in the two highest groups, there is no guidance on the maximum salaries. The joint union submission said it would be `reasonable' for pay in these schools to be capped. The growth of executive headships has also not been fully recognised in leadership pay scales.
Brian Lightman, general secretary of the Association of School and College Leaders, said members were keen to establish principles for assessing heads' pay, rather than coming up with a concrete figure for what the upper limit should be.
"It's a complicated issue - it depends whether you've got a head who is running a single school, or (one who is) taking on a range of responsibilities," he said. "You've got to have a clear rationale about the basis on what (heads are) remunerated for."
The unions' submission echoed concerns voiced by STRB chair Dr Anne Wright, who warned Mr Gove that a cap could inadvertently increase salaries, as heads see it as a target to aim for. The STRB will publish its recommendations in the spring.
IN THE SPOTLIGHT: Heads they win (up to pound;279k)
Headteachers' pay has been in the spotlight since the salaries of a number of school leaders came under fire earlier this year.
Jacqui Valin, head of Southfields Community College in London, was the subject of a media storm after details emerged of her pound;198,000 pay packet.
Mark Elms, head of Tidemill Primary School in London, took home pound;279,230 in 20092010, including pension benefits and payments for extra work.