Concerns have been raised that unqualified teachers are still being used to inspect schools.
There are at least five lead inspectors carrying out inspections on behalf of Ofsted that do not hold qualified teacher status (QTS), it was reported on Friday.
The disclosure prompted warnings from school leaders that it is "essential" for anyone inspecting schools to have the appropriate qualifications and experience.
Last month, Ofsted chief inspector Sir Michael Wilshaw insisted that the inspectorate tries to ensure that inspectors come from good or outstanding schools, and are of a high quality.
But it was revealed that Tribal, a firm which carries out inspections for Ofsted, currently uses five lead inspectors, who are allowed to rate schools, who do not have QTS.
The firm recently sent an email to all its inspectors asking to be informed of their qualifications.
The leaked email, seen by the Times Educational Supplement (TES) said: "No doubt you are aware of the recent media scrutiny into the background of inspectors. In the past this has not been an issue and so we have not asked inspectors to provide us with information about their background.
"Clearly this situation is changing and we need to have accurate records as to whether our inspectors have qualified teacher status (QTS) England."
Russell Hobby, general secretary of the National Association of Head Teachers (NAHT) told the TES: "I don't know how anyone who isn't qualified could provide a meaningful assessment of the quality of teaching.
"Schools' reputations and teachers' careers are made and broken on the basis of these reports. Parents think they are authoritative."
The NAHT recently launched a new website calling on schools to to report their experiences of Ofsted inspections amid concerns that the watchdog is doing little to help raise standards.
The abilities of Ofsted inspectors came under fire last month when it was revealed that former failing headteachers have been recruited by the watchdog to inspect schools.
Governors and ex-school secretaries have also taken on the job of Ofsted inspector despite never teaching a class, according to an investigation by BBC's File on 4 programme.
Ofsted's chief inspector Sir Michael Wilshaw admitted that the watchdog uses some inspectors to look at areas other than teaching, but added that if there are inspectors who have failed as heads, or never taught, then that needs to be dealt with.
Sir Michael also told the programme: "We try to ensure that our inspectors are of high quality, they are recruited from 'outstanding' schools and 'good' schools, and that they know what they are talking about."
A Tribal spokeswoman said it was aware of the five lead inspectors without QTS, which she said represents less than half a percent of the total freelance inspection pool.
"We are awaiting Ofsted's clarification as to whether these inspectors can continue to inspect from 1 September 2012.
"The communication in question went out to all inspectors on our database, including those who are not currently inspecting for Tribal, to check whether there are any others without QTS."
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL) said that statement was "absolutely extraordinary".
"It is essential for the credibility of the inspection service that anyone who is inspecting a school has the appropriate qualifications and experience in that sector.
"It is extremely worrying that a contractor is not aware of its inspectors' backgrounds.
"I have no problem with a lay inspector carrying out a role in inspections, but anyone who is evaluating the teaching profession should be suitably qualified."
An Ofsted spokeswoman said: "Lay inspectors ceased to exist in 2005, however a very small number became additional inspectors.
"Like all inspectors, they have many years of experience in education and inspection and are extremely knowledgeable about schools. All inspectors receive on-going rigorous training."