Heads of failing schools are sitting in judgement on other schools as inspectors, the Office for Standards in Education has admitted.
As Ofsted continues its investigation into an inspector accused of incompetence as a headteacher, it revealed that staff who lead schools into special measures could still become inspectors.
Former teachers in management roles who apply to join the inspectorate have to declare if their schools were judged to be failing by Ofsted. A spokeswoman said: "In such cases, applicants must set out reasons why they still fulfil the requirements to be considered suitable to be trained in inspection skills. Consideration of the applicant's circumstances is then made to determine suitability."
A recently registered inspector, whose school went in and out of special measures while she was head, said that Ofsted does not question applicants about these statements.
Ofsted has banned 43 people from inspecting in 10 years following school complaints or because internal reviews identified repeated irregularities in their reports.
David Bell, chief inspector, has claimed that inspectors are under as much scrutiny as schools.
But Professor Ted Wragg, from Exeter university, said Ofsted procedures needed to be tightened. "We expect that those who inspect have been thoroughly vetted. To become an inspector you should be grilled on your record, so you can earn the respect which is an important part of getting teachers to change their behaviour," he said.
But there should not be a ban on inspectors being drawn from failing schools, many of which contain very good teachers, he said.