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Ofsted has called for the resumption of routine checks on outstanding schools, after 80% of those it re-inspected due to specific issues were downgraded.

England's schools standards watchdog re-inspected 305 schools rated outstanding, after concerns were raised about falling standards.

It said 256 lost their top-level rating as a result.

In 2011, inspectors were stopped from carrying out routine inspections of these top-rated schools.

The move, during Michael Gove's time as England's Education Secretary, aimed to focus resources on the worst-performing schools but was criticised at the time, as it meant hundreds of schools would not be checked at all.

Last year, Ofsted highlighted the issue, saying that as some schools had not been inspected for a decade or more, there was a chance their ratings no longer truly reflected standards at the school.

Some 'outstanding schools not that good'

Ofsted inspection grades challenged

It has been lobbying ministers to reinstate routine inspections every six years for primary and every five or seven years for secondary schools.

Among the 305 "outstanding" schools inspected this year:

166 were judged to be good

76 were found to require improvement

14 were rated inadequate

49 retained their outstanding grade

Ofsted chief inspector Amanda Spielman said: "The fact that outstanding schools are largely exempt from inspection leaves us with real gaps in our knowledge about the quality of education and safeguarding in these schools.

"Some of them have not been inspected for over a decade, and when our inspectors go back in, they sometimes find standards have significantly declined.

"We believe most schools judged outstanding are still doing outstanding work.

"But for the outstanding grade to be properly meaningful and a genuine beacon of excellence, the exemption should be lifted and Ofsted resourced to routinely inspect these schools."