Ofsted has said it cannot disclose further details of antisemitic material found during the inspection of unregistered Muslim schools because of the possibility of future prosecutions.
Education Secretary Nicky Morgan has asked the inspection service to prepare cases against schools suspected of operating beyond the law.
Ofsted head Sir Michael Wilshaw last week reported that inspectors who visited three unregistered schools in Birmingham had discovered squalid conditions, "misogynistic, homophobic and antisemitic material".
A spokesman for the inspection service said this week: "These unregistered schools are now subject to more investigation so we can't provide any more detail that might put a successful prosecution at risk".
While the government's primary concern is to prevent religious extremism that could lead to violent radicalism, Sir Michael has made clear that inspectors are ready to be tough with other unregistered schools teaching pupils under 16.
The Department for Education admitted earlier this year that it had a list of more than 20 unapproved yeshivot teaching boys in Stamford Hill, north London. But its wish to register them has been legally challenged.
In response to a Freedom of Information request from the JC, an official said a number of yeshivot had argued that "they are not required to register as independent schools because their wholly religious curriculum suggests that they are not providing secondary education". Therefore, they fell outside the definition of "school" according to the 1966 Education Act.
Only the courts, the official added, could decide "whether or not a particular institution is, in fact, a school" and the department was still considering the issue.
The government has proposed new legislation which would enable Ofsted to inspect supplementary schools such as cheders and Islamic madrassas and could bring yeshivot into its net. Their educational content would have to be shown not to "undermine" British
values.
Within the Charedi community, differences remain over whether yeshivot should be registered amid concerns over issues including the teaching of same-sex relationships.
