While schools traditionally view Ofsted with suspicion, Her Majesty's Inspectors (HMIs) have long enjoyed a reputation as respected exponents of the inspection regime in England.
Directly employed by Ofsted and, in theory, appointed by the Queen, HMIs are regarded by many headteachers as having impressive professional expertise and authoritative judgement.
But TES has learned that the watchdog does not know how many of its own inspectors have led schools themselves or whether they have worked in a primary or a secondary, let alone how many have hands-on experience of headship of an outstanding school.
"It would be inconceivable that anyone who has not recently had leadership experience in a successful school could pass judgement in an inspection," said Brian Lightman, general secretary of the Association of School and College Leaders (ASCL).
The discovery was prompted by an enquiry from experienced "superhead" Stephen Ball, who has led four schools and directed policy for the Grace chain of academies in the Midlands during his 18-year career in school leadership.
None of the four inspections he has undergone as a head were led by an inspector with experience of secondary headship, he says. In recent months, a number of his peers have reported that their schools' inspections were led by HMIs with no relevant experience.
To put this to the test, Mr Ball submitted a Freedom of Information request, asking how many HMIs had experience of being a secondary head, and how many of those had led an outstanding school. The answer? Ofsted "does not hold the details".
"Secondary heads and academy principals need to be reassured that their work is judged by people who understand its complexity," Mr Ball said. "Training as a good head of department or a primary school leader on the framework is no longer adequate. Heads don't fear judgement, but they expect to be judged by people who have experience as well as a theoretical training."
Mr Ball's concerns are shared by NUT general secretary Christine Blower. "For Ofsted not to know exactly what previous experience their inspectors have is woeful," she said. "Teachers have long been concerned about the level of expertise of many inspectors. Some appear to have so little knowledge of life in the classroom it is unlikely that they have been near a school for some time."
Concerns about the consistency and quality of inspectors are also evident in the primary sector, according to Russell Hobby, general secretary of the NAHT heads' union. "If you get former secondary heads inspecting early literacy, you get the same sort of problems. If you have never done it yourself, ensuring the accuracy of your judgement may be difficult."
An Ofsted spokesman insisted that "the majority" of HMIs have experience of leadership in schools, colleges or local authorities. "They are all education professionals with knowledge of the classroom and subject- specific knowledge where needed, and relevant skills in primary, secondary, special schools or PRUs (pupil referral units)," he said.
"Increasingly, inspection teams contain serving practitioners, including currently serving headteachers and deputy headteachers of secondary schools, which adds a further dimension to the inspection expertise of the team . They will only be recruited if they have been excellent leaders and managers themselves," the spokesman added.
The number of HMIs by field:
24 - Early years
2 - Equalities and human rights
20 - Knowledge management
82 - Learning and skills
5 - National adviser
141 - Schools
44 - Social care
318 - Total.