The proportion of Ofsted reports resulting in formal complaints by schools has more than doubled over the past two years, new figures show.
An official complaint was lodged for one in every 20 school inspections completed between April and September 2010, with the percentage of complaints that were upheld by the watchdog also increasing for the second successive year.
Heads' union the NAHT warned the figures were only "the tip of the iceberg", with its members expressing widespread concern about the variable quality of inspectors, while other teaching unions blamed the increase on the introduction of Ofsted's controversial new inspection framework in 2009.
In the first half of 2010-11, 151 complaints were received, amounting to 5.6 per cent of the 2,685 inspections carried out - up from just 2.6 per cent in 2008-09.
The proportion of complaints upheld by the watchdog also increased by more than a third to 13.9 per cent during the same period.
The complaints referred either to inspectors' overall judgments or to specific elements of a report with which the school was unhappy.
Russell Hobby, NAHT general secretary, said: "Inspection teams vary widely in both quality and approach. Sometimes the goalposts move so frequently that even the inspectors are left behind.
"A stable and slimline inspection framework, used by inspectors with credible recent experience in school leadership, would help.
"However, recorded complaints may only be the tip of the iceberg. Heads are not always confident a complaint will be taken seriously, and a bad inspection process can be so draining that all the school wants to do is put it behind them."
Nansi Ellis, the Association of Teachers and Lecturers head of education policy, blamed the rise in complaints on the inspection framework introduced in 2009, which included a greater focus on raw data, shorter notice periods before inspections and a doubling of time spent observing lessons.
"Schools were finding aspects of the framework very challenging, and were particularly concerned about the new way Ofsted was assessing safeguarding and about little things having a disproportionate impact upon the overall grade schools were given," she said.
"ATL members are also often unhappy with the inspectors themselves and complain about inspectors not having enough knowledge or expertise of the age groups, particularly early years, they are inspecting."
NUT general secretary Christine Blower said: "Some headteachers have been incensed by (chief inspector Christine Gilbert's) claims that most schools are supportive of inspection and have good experiences.
"Both the NUT and NAHT campaign to encourage members to use the complaints system more and be supported in these efforts by their union. This could also have had an effect. The fact that a growing number of complaints are upheld would suggest that there must have been some very serious breaches in inspection protocol."
An Ofsted spokeswoman said the increase could be down to "heightened awareness of our complaints process" following a 2009 public consultation.
The "vast majority" of reports are not disputed, she added. Feedback about the new arrangements had been "overwhelmingly positive", with 95 per cent of heads surveyed describing Ofsted's findings as "fair and accurate".
"The new framework ensures closer engagement of school senior managers," she said. "This enables them to understand and be aware of the evidence underpinning the judgments made and the reasons for the recommendations."
- Original headline: School complaints over Ofsted double in two years