A school has been ordered by the government to stop segregating male and female pupils.
The Imam Muhammad Adam Institute in Leicester was rated "inadequate" by Ofsted inspectors who observed the practice during a visit in March.
The independent school, which has 346 pupils, was warned it was in breach of the Equality Act of 2010 by teaching girls and boys at separate sites.
Inspectors also criticised pupil safety at the Islamic faith school.
Ofsted's report said the school must end its "unlawfully discriminatory practice of strict segregation by sex".
It said: "Pupils in Years 5 and 6 and in the secondary provision are segregated by sex for both their lessons and their social time. They are taught at separate sites and do not have the opportunity to mix.
"The policy causes detriment to pupils in that they are unable to mix or to learn together when at the school. This limits pupils' social development and prevents them from preparing effectively for life in British society."
Inspectors also found a science room at the secondary girls' school site was unsafe and a fire exit had not been properly checked.
The school was ordered to end its "strict segregation" policy immediately and "rapidly" improve pupil safety.
The report comes after an Islamic faith school in Birmingham was found guilty of sex discrimination in 2017.
A bid by the Association of Muslim Schools to appeal against the ruling was rejected by the Supreme Court.
The BBC has invited the Imam Muhammad Adam Institute to respond to Ofsted's criticisms.