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Thousands of state schools across England are segregated along ethnic or social grounds, according to research.
More than a quarter of primary and four in 10 secondary schools are ethnically divided, the social integration charity, The Challenge, found.
It says almost a third of primary and a quarter of secondary schools are segregated along socio-economic lines.
The Department for Education says all schools are expected to promote social integration and British values.
Researchers from The Challenge - working with the iCoCo Foundation and SchoolDash - measured how segregated a school was by comparing its numbers of white British pupils and those eligible for free school meals with those of the 10 schools closest to them.
They used official statistics for the years 2011 to 2016, examining more than 20,000 state schools.
Areas singled out for particular concern were:
◾Kirklees in West Yorkshire
◾Lancashire as a whole, but especially Blackburn with Darwen
◾Rochdale in Greater Manchester
◾Birmingham
The researchers regarded a school to be "segregated" if the proportion of ethnic minority pupils or pupils on free school meals was very different to the proportions at the neighbouring schools.
They found secondary schools were more likely to be segregated by ethnicity than socio-economic status, while primary schools were more likely to be segregated along socio-economic lines.
Primary faith schools were more ethnically segregated than those of no faith (29% against 25%) when compared with neighbouring schools, the study found.