Headteachers working in challenging schools are more likely to be criticised by Ofsted for being bad leaders, according to a study published today.
It is much easier for heads in affluent areas, with brighter children, to be rated outstanding by the schools watchdog, according to analysis by the Association of School and College Leaders.
John Dunford, the union's general secretary, said the apparent bias showed inspections were not carried out on a level playing field.
More than four out of five schools that received an "outstanding" for leadership in 200506 were in average or better socio-economic areas, the union's figures show.
In 88 per cent of schools with outstanding leadership, pupil attainment was also average or better when they had started at the school.
"A combination of a socio-economic disadvantage and below-average attainment on entry was a characteristic of only 7 per cent of schools judged (by Ofsted) to demonstrate outstanding leadership," the union's report said.
Gordon Brown has called for the best teachers to move to the toughest schools in a bid to eradicate failure. But skewed inspections would put talented teachers off taking up those posts, Dr Dunford said.
"Careers are made and lost on Ofsted inspections," he said.
Philip O'Hear, principal of Capital City Academy, in Willesden in west London, said: "It is true that more is required of leaders in challenging schools to show that what they are doing is outstanding. It is an issue that needs to be highlighted. But it is important that we have very a demanding inspection framework."
Too many school leaders found their ratings changed from inspection to inspection, when the only difference was the school's circumstances, the report said.
An Ofsted spokeswoman said: "Inspectors take full account of a school's socio-economic context when evaluating the progress made by its pupils. Inspectors see outstanding leadership in a wide range of schools. Ofsted believes that leadership should be effective no matter where a school is located."
The ASCL paper also called for an end to inspections for high-performing schools.