Inspectors have carried out more than 220 emergency inspections of private schools in England over the past five years, with the number of visits taking place in 2012-13 almost double that of the previous year.
New figures obtained by TES reveal that emergency inspections ordered by the Department for Education rocketed to 62 in the 2012-13 financial year, up from 35 the year before. In total, 177 schools have been subject to 224 emergency inspections since 2008, with 39 schools later closing down.
Inspectorate Ofsted has refused to provide a breakdown of the reasons why the inspections took place, but last month it revealed that fears over religious extremism being taught in schools were behind a "small proportion" of them.
Emergency inspections can also be triggered by concerns over safeguarding, student welfare and the suitability of staff.
The Association of Teachers and Lecturers, the biggest teaching union in the private sector with more than 20,000 members, described the figures as "worrying". Adrian Prandle, the union's policy adviser for inspection, called on Ofsted to release more information about the emergency visits.
"The numbers look worrying, but we need to know the reasons why (the inspections are happening)," he said. "If there are serious things going on in some schools, it's right something is being done. But that doesn't always mean that Ofsted is the right thing."
An Ofsted spokesman said: "The purpose of the emergency inspections is to investigate concerns raised about an independent school. Concerns raised can cover a number of different areas including welfare, health and safety, quality of education (and) suitability of staff, as well as a range of other matters."
The Independent Schools Council, which represents 1,300 of around 2,000 private schools in the UK, told TES that none of the emergency inspections carried out by Ofsted took place at its schools, which are instead monitored by the Independent Schools Inspectorate.
Brian Lightman, general secretary of the Association of School and College Leaders, said the schools visited are likely to have been "smaller independent schools, which are not as regularly inspected".
The information on emergency inspections was released by Ofsted after a request by TES under the Freedom of Information Act. However, the watchdog refused to disclose the names of the schools visited or provide a breakdown of the reasons for the inspections.
Three of the emergency inspections are known to have taken place since January at Stanbridge Earls School in Hampshire, which teaches students aged 10-19 who have special educational needs. The inspections were ordered because of concerns about safeguarding after it was found that the school, which charges fees of up to #163;40,000 a year, had failed to protect a vulnerable girl who claimed she had been raped by fellow students on the school site.
After an internal inquiry, Ofsted admitted that it had "failed to get underneath concerns at the school" during earlier visits. In July, inspectors ruled that the school, which has since announced that it will close in December, "cannot yet be considered a safe institution".
A Department for Education spokeswoman denied that there had been a surge in the number of emergency inspections it orders. "It is incorrect to say there has been a sudden increase in emergency inspections," she said. "Last year saw the same level of inspections as five years ago.
"We can ask Ofsted to make emergency inspections of independent schools when concerns or complaints are raised with us. Where schools are failing to meet these standards we will not hesitate to take action so (the situation is) rectified."
CAUSE FOR CONCERN
Number of emergency inspections of private schools
60 - 2008-09 financial year
23 - 2009-10
44 - 2010-11
35 - 2011-12
62 - 2012-13