Ofsted’s chief investigator has said that the number of children being taught in illegal schools in England is in the thousands — much more than was previously thought.

Sir Michael Wilshaw is pushing for the government to focus more on the rules concerning home-based education, saying many parents are taking advantage of the rules, as made evident by the growing number of illegal schools throughout the country.

In just the last five months, over 100 schools believed to be unregistered have been found out by inspectors. Some of them are teaching children in “unsafe and unhygienic premises” with staff members who have not gone through a proper background check.

Wilshaw said that many of the unregistered schools had been taking advantage of the freedoms that are offered parents who choose to home-school their children, reports Hannah Richardson for the BBC.

The crackdown of these types of schools began in January with a specialist taskforce of seven experienced inspectors. Wilshaw said that since that time, new cases are being reported each week.

Wilshaw discussed the growing problem in a letter to education secretary Nicky Morgan last week, saying that the evidence collected by the inspectors show a clear link between the rise of unregistered schools and the number of children counted as being home-schooled in recent years.

“I have previously voiced concern that many of those operating unregistered schools are unscrupulously using the freedoms that parents have to home-educate their children as a cover for their activities. They are exploiting weaknesses in the current legislation to operate on the cusp of the law. Many are charging parents thousands of pounds to send their children to these unregistered schools.”

He went on to say that many of these schools are offering children a substandard education, which not only places them at risk, but also undermines efforts made by the government to make sure schools are promoting British values like “tolerance and respect for all.”

A separate investigation by the BBC last year found a 65% increase in the number of children who were being reported as home-schooled in the country over a span of six years. However, that number is thought to be an underestimation, reports Sally Weale for The Guardian.

Warning notices were sent to seven such schools throughout London, Birmingham, Luton, Wolverhampton, and Staffordshire in April. In all, 350 children were found to have been attending these schools.

Findings at the schools included fire hazards, staff and volunteers who had not received background checks, and unsafe buildings. One school was storing chemicals and chemistry equipment in an unlocked cabinet in the same room where students ate lunch.

A spokesperson for the Department of Education noted that the government was taking several steps in order to be sure that the rules concerning home education were strict. However, the department said that at the same time, the rights of parents to determine both how and where to educate their child must be protected.

“Nothing is more important than keeping children safe, and councils have clear powers to take action where there are concerns regarding a child’s wellbeing,” the spokesperson said. “We have given new resources to Ofsted to investigate unregistered schools, and to prepare case files for prosecution by the CPS.”