Some independent faith schools are "flouting British values" by continuing to enforce gender segregation, Government inspectors have found.
A report by Ofsted on Rabia Girls' and Boys' School in Luton found staff insisted on using a dividing screen across the middle of the room to separate men and women during a meeting with inspectors.
Senior inspectors visiting the Muslim faith school also gathered evidence that male and female staff are regularly segregated during whole-school staff training sessions. It meant male staff would sit in one room while the session was simultaneously broadcast to females in another part of the school.
The findings were revealed in a letter from Ofsted Chief Inspector for England and Wales Sir Michael Wilshaw to Education Secretary Nicky Morgan relating to inspections carried out earlier in April.
In his report to the Secretary of State, also the Equalities Minister, Sir Michael said: "Despite the changes introduced in 2014 by the Department for Education (DfE) to strengthen the independent school standards in relation to fundamental British values, it is clear that these are not being followed by some independent schools. Indeed, it is my view that these revised standards are being actively undermined by some leaders, governors and proprietors.
"Inspectors will remain vigilant in ensuring that such behaviour, which clearly flouts the requirement to promote British values, is identified and reported. Any form of segregation, without a good educational reason, is likely to lead to an inadequate inspection judgment for leadership and management."
Sir Michael s aid inspectors were so concerned about the behaviours modelled by leaders at Rabia school that they informed the proprietor it would remain in the "inadequate" category despite improvements being made elsewhere.
He said: "This sort of behaviour manifested by the leaders of this school clearly does not conform to the spirit of the equalities legislation which underpins the spiritual, moral, social and cultural standard."
The report is the latest to highlight inequalities in some faith schools.
In November, Sir Michael wrote to Ms Morgan with concerns about nine inadequate schools, including the Darul Uloom Islamic High School in Birmingham, after inspectors found the only way a woman governor could contribute to meetings was through a doorway.
At the Al-Ameen Primary School, also in Birmingham, pupils were not protected from "reading inappropriate literature about extremist, sexist or partisan views", the report found.
And pupils at the Christian faith Cornerstone School in Epsom said they had "a limited view of the world because their education was not providing them with sufficient opportunities to learn about people with differing backgrounds and perspectives to their own".
A DfE spokesman said the school has been referred to the Equalities and Human Rights Commission to decide whether it breached equalities legislation.
He said: "It is completely unacceptable for women to be treated less favourably than men, and the advice note we have received from Ofsted on Rabia Girls' and Boys' School is extremely concerning.
"We will consider carefully the inspection report on the school to determine what action to take against any potential breaches in the independent school standards."