The government has launched a consultation into changing the definition of full-time independent schooling so it can clamp down on illegal schools.
In the consultation report, it notes that the 2008 Education and Skills Act defines a "school" as an establishment offering a broad education for at least 18 hours or more per week.
However, this definition means illegal schools where pupils are offered a very narrow curriculum of religious instruction evade registration as independent schools, on the basis that they are not technically schools.
This loophole means such institutions also avoid inspection from Ofsted – and the inspectorate has made calls for the change.
The consultation document went on: "It has been contended, and the department [for education] accepts, that the curriculum offered at settings which is a single discipline or is very narrow in nature, does not constitute education suitable to the requirements of children of compulsory school age."
"This means that the settings providing such a narrow education are not schools, and therefore cannot be registered or regulated."
The department has decided that institutions will need to register if they are operating for 18 hours or more over seven days a week rather than from Monday to Friday.
The definition will apply to institutions operating during the normal school day and will exclude intensive sports clubs and other extra-curricular activities.
The document said that "if the setting operates out of normal school hours, in the evenings or at weekends – such as intensive sports training, or instrumental music tuition, or ballet, for example" then the new regulations will not apply.
"It is possible that such provision may exceed the threshold of 18 hours per week; but the way it is organised does not prevent the child from also attending school," the document said.
The proposed changes aim to address "the issue of the nature of the education provided" at illegal schools.
It would require registration of settings offering an intensive religious-only education, which would currently have difficulty in meeting independent school standards owing to the narrow education provided.
The document added: "Ministers currently take the view that all such settings, operating full time, during the school day, for compulsory school age children, should register and meet applicable independent school standards so that children of that age receive a suitable education if that is conducted entirely in an educational setting or the hours of operation of such settings make home education effectively impossible."