Some schools rated outstanding may no longer be as good as their rating suggests, Ofsted has said amid official criticism of its work in England.
A National Audit Office report found 1,620 schools, mostly outstanding, had not been inspected for six years or more, and 290 for a decade or more.
Outstanding schools were decreed exempt from routine inspections in 2011.
Ofsted bosses said there was no way of telling if these schools had since fallen into a "mediocre" category.
Although, inspections can be triggered at any school if a safeguarding concern is raised, or if there is a significant drop in results.
It no longer goes into these top-rated schools on a regular basis.
Ofsted's director of corporate strategy, Luke Tryl, said: "What we can't tell is if the levels of education in those schools judged outstanding 10 years ago are the same or whether it has changed to become middling, or mediocre or coasting."
When asked by reporters if he was saying that some "outstanding schools aren't really outstanding", he replied: "Yes."
However, many schools will have their "outstanding" label highlighted on their websites and on banners outside their premises.
And many parents base at least their initial views of such schools on these Ofsted rankings.
