The Department for Education has warned co-educational schools they could be infringing equality rules if they separate boys and girls, even for religious reasons.

The DfE published new guidance on Thursday following a Court of Appeal ruling last year against a state-aided Muslim school that said it was wrong entirely to segregate pupils by gender.

A separation of boys and girls in a mixed school which denies them the chance of social or educational interaction could be seen as to the disadvantage of either sex, the DfE warns.

Such a policy could be unlawful unless it complies with permitted exceptions, the DfE says.

Where there is separation, the school’s leadership would be expected to justify it to Ofsted.

A number of Jewish schools separate boys and girls for Jewish studies - but the DfE guidelines do not go into specific circumstances.

However, one school, Hasmonean High, which teaches boys and girls on separate sites, is planning to split them formally into two schools as a result of the court ruling.

Giving an example, the DfE says it would be unlawful to offer food technology to girls only and metalwork to boys.

It would be permissible to run sex education classes separately for boys and girls, the education authorities say.

A school could also justify running girls-only classes in science, maths and technology if it wanted to encourage more girls to pursue these subjects where their participation was low.

“There may be occasions where a mixed school can lawfully separate boys and girls if the separation can be considered negligible in its effect on the ability of pupils of both sexes to mix, socialise or learn from and with each other,” the guidance says.