A Muslim free school accused of imposing strict Islamic practices, such as segregated classrooms, has closed following an inspection by Ofsted.
The BBC understands Ofsted's findings were so damning that the acting head of Al-Madinah, in Derby, had little choice but to shut it down immediately.
The school said the move was due to a "health and safety issue" but expected it to reopen in the "very near future".
Ofsted said it could not disclose its concerns until the inspection ended.
It added it had "made some findings and shared them with the principal". The second day of the inspection is taking place later.
In a statement on the school's website titled "short term closure", interim principal Stuart Wilson said: "Owing to a health and safety issue, I have taken the decision to close the school... until I am confident that all children are safe on site.
"As parents, you will be informed directly, and on the website, when you are able to send your children back to school...
"Assuring you that we have your children's best interests at heart."
In a series of newspaper reports unnamed former staff members of Al-Madinah, which opened as a free school in September last year, had alleged that girls were forced to sit at the back of the classroom.
Unnamed female staff members have also claimed they were forced to conform to a strict dress code including wearing a head scarf or hijab - whether or not they were Muslim.
When it opened Al-Madinah claimed to be the first Muslim ethos, all-through [reception, primary and secondary] free school in the country.
The school's first head teacher, Andrew Cutts-Mckay, left the school after less than a year in the job.
Last week, the interim principal told the BBC that he had not received any complaints from colleagues regarding the dress code and that pupils were not being segregated, with girls and boys being treated equally.
Ofsted is not the only organisation with concerns about Al-Madinah.
The Education Funding Agency - from which the school gets its public funding - is investigating alleged financial irregularities.
In a statement, the Department for Education said it was already investigating the school before the allegations became public.
It said: ''We discussed the problems with Ofsted and it launched an immediate inspection. We are waiting for Ofsted's final report and considering all legal options."
The school's closure is likely to be embarrassing for Education Secretary Michael Gove, who introduced free schools in 2010 in an effort to raise standards in education.
Free schools are state funded but operate outside local education authority control and can be set up by parents and community groups in England.