More than 100 illegal schools have been discovered by Ofsted, most of which are religious.
The school inspection body said the total was much higher than had previously been thought and comes after a crackdown on unregistered and illegal schools, according to the BBC.
The majority of the illegal schools discovered were Islamic or Jewish and are run by faith groups, said Ofsted's chief inspector Sir Michael Wilshaw. The revelation was made after an investigation by a team of seven inspectors alongside the Department for Education.
The team visited the sites of suspected illegal schools and 350 children were discovered on their premises in the last month.
"The evidence they have gathered so far during this short period firmly reinforces my belief that there are many more children hidden away from the view of the authorities in unregistered schools across the country than previously thought," said Wilshaw.
He added that such schools put children at risk and undo the government's drive to ensure all schools promote British values. The worry, Wilshaw said, was that children would be exposed to extremism and radicalisation without the government being able to intervene.
Parents are free to home educate their children but unregistered schools use this as cover for their activities.
Under the current law any school that offers more than 20 hours of lessons a week must be registered. However this may change as Wilshaw promised to take action against those who run illegal schools.
A Department for Education spokesman said nothing was more important than keeping children safe and added local councils have powers to take action where there are concerns.
"We have given new resources to Ofsted to investigate unregistered schools, and to prepare case files for prosecution by the CPS," the spokesman said.
"We have consulted on new measures to protect children in out of schools settings offering intensive education. We received a large number of responses, which we are now considering, and will make a further announcement in due course."
The government's promised regulation of out of school settings has raised fears among Christians that churches and youth groups could be inspected by Ofsted.
Shadow education secretary Lucy Powell said the government's policies had allowed dangerous practices to emerge in schools.
"The Tories' education policy has led to a fragmented schools system lacking robust local oversight to spot and tackle serious problems early on," she said.
"As a result, many children are dropping off the radar or ending up in illegal, unregistered schools for months or years, where they are at risk of being exposed to harm, exploitation, or the influence of extremist ideologies."
