Labour has accused the government of failing to take fast enough action to close down illegal schools.
There are 21 institutions being investigated in England by the Department for Education, according to a ministerial statement.
The statement from children’s minister Edward Timpson says “robust steps” are being taken to tackle unregistered schools.
But Labour’s Lucy Powell said ministers had “dragged their feet”.
The issue of illegal schools was raised by Ofsted last year.
Inspectors warned of pupils being taught in schools that were not registered, inspected or subject to any kind of formal accountability – and Ofsted chief Sir Michael Wilshaw called for urgent action to get them closed.
These are schools which are running as full-time places of education, rather than classes providing extra lessons.
In November, inspectors reported that they had uncovered 15 such illegal schools and then in December a further three schools were found operating outside the law.
The only one to be publicly identified was Bordesley independent school in Birmingham, which had already been closed down.
Labour has been pointing to this month’s ministerial response which said there were 21 current investigations into institutions that might be “unregistered independent schools”.
“However, this number often changes as new settings are established and existing ones close, or are confirmed not to be operating as schools,” continued Mr Timpson’s statement.
“We have taken robust steps to tackle unregistered schools and improve safeguarding which includes being clear it is a criminal offence to operate an unregistered independent school.
“All these institutions have been warned that it is a criminal offence to operate as an unregistered school. We are asking Ofsted to visit and to prepare cases for prosecution as necessary.”
In November, Ofsted wrote to the education secretary saying: “The arrangements for closing down unregistered schools are inadequate.
“Too many children remain at significant risk of harm. I will continue to do all that I can to identify and inspect unregistered schools.”
Lucy Powell, Labour’s shadow education secretary, commenting on the investigations, said: “Ministers have a basic responsibility to ensure that children are kept safe, and yet despite warning after warning, they have dragged their feet on this very serious issue, leaving children in unregistered schools where they could be in harm’s way.
“It is extremely worrying that Ofsted remain concerned that the number of children being educated in unregistered provision in parts of the country far exceeds the number that is currently known by the government.
“Without a doubt, the dangerous lack of local oversight of our schools system, created by the Tories’ education policy, is putting children at risk.”
But a spokesman for the Department for Education said there were already steps announced to “crack down” on such “illegal and unsafe” unregistered schools.
“Last month we announced an escalation of Ofsted investigations into unregistered schools, with additional inspectors dedicated to rooting them out, a new tougher approach to prosecuting them and a call to local authorities to help identify any settings of concern.
“Our tougher stand against illegal schools will help prevent children from falling into the grasp of extremists.”