The head of Ofsted Sir Michael Wilshaw said that he was “extremely concerned” at the number of unregistered schools after more than 100 have so far been identified by inspectors.

In a letter to Education Secretary Nicky Morgan today, he said that “there are many more children hidden away from the view of the authorities in unregistered schools across the country than previously thought”.

An Ofsted spokesman confirmed that the figure includes some Jewish institutions. The Department for Education has a list of 23 unregistered yeshivot, while Hackney Council has put their number as high as 29.

“Suspected new cases” every week were coming to the attention of a special cial taskforce of Ofsted inspectors which was set up the beginning of the year to investigate the problem, Sir Michael said.

He was “extremely concerned about the number of children and young people attending these schools who may be at significant risk of harm and indoctrination”.

Evidence gathered by the taskforce reaffirmed his belief that there was “a clear link between the growth of unregistered schools and the steep rise in the number of children recorded as being home educated in England over the past few years,” he said.

Unregistered schools were “exploiting weaknesses in the current legislation to operate on the cusp of the law”.

Many were providing “a sub-standard education, placing children at risk and undermining the government’s efforts to ensure that all schools are promoting British values, including tolerance and respect for others”.

It is illegal for schools teaching children under the age of 16 for 20 or more hours a week not to register with the education authorities.

One independent Charedi primary school in Hackney which taught no English was shut on the orders of the DFE in February.

But an argument has been put to the DFE that under the current legal definition yeshivot do not count as schools and are therefore exempt from registration.