Hundreds of children are being taught in “squalid” religious schools enforcing gender segregation and a “narrow curriculum”, the education regulator’s chief inspector has warned. He said no one had ever been prosecuted for running the unregistered schools, and more needs to be done.

Sir. Michael Wilshaw said that Ofsted inspectors have identified 15 unregistered schools in a letter to Nicky Morgan, the Education Secretary. He called for greater powers to close them down swiftly, and drew particular attention to authorities in Birmingham for failing to act.

He wrote that 800 pupils had been found in school with “Squalid conditions, including three single mattress covered in filthy sheets in one room and no running water in the bathroom area”.

At the school there was “Clear evidence of segregation, with separate classrooms for boys and girls”, “no evidence of appropriate vetting checks being carried out on staff”, and pupils were being taught a “narrow curriculum that was failing to prepare them for life in modern Britain.”

The institutions are thought to be both non-faith and faith schools, including Bordesley Independent School, which caters for Muslim children in Birmingham, the Telegraph report. Bordesley has since been shut down.

Authorities in Birmingham were also at the centre of the so-called ‘Trojan Horse’ plot, were several secular state schools were taken over by hard line Islamists.

Sir. Michael said, “local authorities are not acting swift enough” and “too many children remain at significant risk of harm.” Adding: “I will continue to do all that I can to identify and inspect unregistered schools.”

In the letter, he tells of how inspectors were only able to gain access to one school when accompanied by police:

“Following each previous visit, [inspectors] reported the serious concerns they had for the welfare of the children being educated in this unregistered school. At the most recent visit, inspectors were intentionally obstructed from entering the premises for an hour. “During this time, inspectors observed a number of female students to be on the premises. When inspectors finally gained entry, the girls had left the building by an alternative exit. The staff on site informed inspectors that these girls were attending a local library and they would return later in the day. “They did not return and the staff were unable to account for their whereabouts for the direction of the inspection visit.”

Arrangements for closing down unregistered schools are “inadequate” he said. Adding: “I understand that no individual has ever been prosecuted for operating an unregistered school.”