Ofsted has failed to ensure school inspection judgements are reliable, with some inspectors basing their decisions on a “narrow range of data”, one of the watchdog’s leading officials has admitted.

National director for schools Sean Harford has publicly acknowledged for the first time that the inspectorate does not “directly [ensure] that different inspectors in the [same] school on the same day would give the same judgement”.

The unprecedented statement, issued in response to a critical blog about the watchdog by leading headteacher Tom Sherrington, reveals that Ofsted is carrying out “reliability testing” in pilot inspections this term, in order to assess consistency between different teams of inspectors.

The “weakest” inspectors, Mr Harford writes, “have been guilty of using the published data as a safety net for not making fully rounded, professional judgements”.

Stephen Ball, principal of New Charter Academy in Greater Manchester, said Ofsted’s admission was “breathtaking”, adding: “It is astonishing and deeply troubling for an organisation that has one fundamental job - to judge the quality of education in schools.”

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Related stories:

‘Ofsted’s criteria for “outstanding teaching” are outstanding nonsense, and here’s why’ - January 2015

Tristram Hunt: Ofsted must move beyond ‘box ticking and data dependence’ - January 2015