A senior Ofsted official has said that it is "harder" for schools with lower-ability intakes to gain "good" or "outstanding" judgements from the watchdog, TES can reveal.
The admission from Robert Pike, Ofsted's chief statistician, is being seen as a further blow to the inspectorate's claims to offer objective judgements on the quality of teaching and learning in schools.
Mary Bousted, general secretary of the ATL union, said: "The way Ofsted makes judgements on data means they simply compare less privileged schools to more privileged schools. [Less privileged schools] are always on a losing streak and an uneven playing field."
Mr Pike also states that it is "probably easier" for "schools with advantaged intakes" to receive Ofsted's top two grades. He makes his comments in a letter, seen by TES, to a deputy headteacher who has alleged that the inspectorate shows "extreme bias" against schools with lower-ability pupils.
Heads' leaders are calling for the watchdog to give context more importance when making judgements on schools.
Mr Pike's letter was sent to the founder of the National Association of Secondary Moderns, Ian Widdows, who is concerned about the vast disparity between the Ofsted judgements received by his members and those given to selective grammars.
Controversially, the Ofsted official says grammars' higher ratings may be because their teaching is "more effective". Mr Widdows, deputy headteacher at Giles Academy in Lincolnshire, said the claim was an "insult" to teachers in other schools.