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".